Under my system, a specialized magus might have for example an Affinity in and Puissant Ignem, a Minor Magical Focus in burning things, and some other boosting-virtue (say, Cyclic Magic). He will have a score of 6 (112.5 XP) in Ignem, and say 4 (50 XP) in Creo, and let's say Stamina +3. All in all, he has a Casting Total of +22 [3 (Sta)+8 (Ig)+4 (Cr)+4 (focus)+3 (cyclic)]. That's more than enough to cast a Pilum of Fire, and even an Arc of Fiery Ribbons in most environments. Magi are still far, far more powerful than mundanes, even initially.
Besides, one of the motivating factors is that this change will encourage magi to acquire various bonuses. A powerful magus wouldn't be one that simply increased his Arts. Instead, a powerful magus would be the one that is wise in the use of natural correspondces to enahnce his magic (Ceremonial Casting), has mastered the occult art of utilizing the deep mystic resonances of Form & Material (Potent Magic) and constructed a Talisman to enhance them further (Talisman Attunement). He has uncovered the long-forgotten secrets of Gorgiastic Magic, and was initiated into the secrets of Theurgy... Being a powerful wizard requires more than increasing your Arts, it requires actually incorporating occult secrets and traditions into your work - from very generic Hermetic techniques (talisman attunement, ceremonical magic) to the esoteric (potent spells) and even exotic secrets (Theurgy) and the long-forgotten great magics of the past (Gorgiastic Magic). The result are elder magi that are far more colorful and interesting than "I maxed out my Ignem, arrg!".
By my calculations, an old magus would be able to have a Casting Total in excess of 90. That's still too much, really, as this is before various bonuses, not to mention Wizard's Communion. Compare this total to the Might of the powerful demons and angels, and you'll see why. Compare it to the most powerful spells known to the Order, in the published spells, and you'll see why.
If the game progresses far enough, PC magi could reach impressive might. So far dwarfing mundanes that it isn't funny. That this is true despite cutting the Arts by 2, effectively, speaks volumes on the excessive power scale of magi in core ArM.
Consider a core ArM5 wizard. His top Arts, the book tells us, are no more than 40 (820 XP). At the rate of 30 XP per year, and assuming he starts from a score of 10 (55 XP) this translates to a gain of 765 XP over 120 years, i.e. 25.5 years or 0.2125 of his post-Gauntlet life. The ArM5 estimate of 30 XP per year is a bit low in my (limited) experience, but even with it it's entirely reasonable for a magus to have this much XP in one or perhaps even two Arts (it would still leave him more than half his time for other persuits). And this without considering Affinity, Puissance, or the various Quality-enhancing virtues (I definitely think the 30 XP per year is too low; 40 XP per year sounds better, IMO, perhaps even more).
His casting total could easily be +125 [5* (Sta) +40 (Tech) +40 (Form) +40 (Focus)], and will probably be far greater due to virtues, mysteries, a talisman, ceremonical casting, and so on. This allows him, for example, to face off an opponent with Faerie Might 75 - a massively powerful faerie lord - and cast a level 45 spell with penetration just on the raw power alone. Add in Penetration and the various bonuses, not to mention an arcane connection and some sympathetic penetration bonuses, and you easily see that elder magi are simply unstoppable in the setting. They can kill, dominate, or otherwise overcome the most powerful faeries, demons, or angels, and can wield spells of 25th and higher magnitude, enough to demolish entire armies, kingdoms, and cities with a single spell.
[* Stamina of +5 and Intelligence of +5 are extremely easy to manufacture, at these power levels, given sufficient raw vis - so I'm assuming they were.]
Now consider a magus following my system.
An Ability at 20 costs 1050 XP, which is close to the core Art at 40 (820 XP) after an Affinity (1230 XP). Actually, 1230 XP provides for an Ability at 21, so let's work with that figure. So we have two arts at score 21, with two Affinities. Let's add two Puissance Art as well, bringing each to 23. Let's assume a Magical Focus as well, and Potent Magic (perhaps both through initiations); and let's assume a Magic Theory of 10 (modest, I think, for elder magi that can use it) so his Potent Spells acquire a +10 bonus. He surely has an attuned talisman too, let's give him another +5 from that. This alone would traslate to a casting total of +87 [3 (Sta)+23 (Tech)+23 (Form)+23 (Focus)+10 (Potency)+5 (Talisman)]. Add in Artes Liberales and Philosophiae from Ceremonial Casting, plus a +5 bonus for material items used in ceremonies (as per the Jerbiton section of Societas), plus a positive Aura, plus some other virtues or mysteries (like, say, Names of Power)... you fairly easily end up with a Casting Total in excess of 90, probably even in excess of 100. And we haven't even considered raw vis, or included using Confidence...
The result is that such magi are still very powerful. Powerful enough to face even a powerful faerie (able to cast a level 15-20 spell with penetration against MR 75; add in skill at Penetration and spell mastery and they can field level 25 spells or more even without arcane connections, which are effective at fighting but still not "I'll kill you in one spell" uber-powreful). Powerful enough to cast the Order's most powerful spells single-handedly, and with some penetration left. Powerful enough to devise spells that will destroy entire cities, armies, perhaps even kingdoms. But this time, at least this level of power takes effort and greatness - two affinities, two puissant arts, a magical focus, potent magic - all heavy investments.
The typical archmagus will probably not be min-maxed to such a degree, but he surely wouldn't be very far off. He'll still have considerable Art scores, he'll still acquire various virtues, mysteries, and bonuses. The Order is still powerful, immensly powerful.
Finally, consider young magi, under my system. As noted above, a starting specialist magus can very well start with a Pilum of Fire or even Arc of Fiery Ribbons (level 25 spells). With a +22 casting total, he's in principle even able to cast a Ball of Abysmal Flame (level 35 spells) with a bit of luck and fatigue. I think that's more than enough for a magus to stand out from the mundanes.
A magus that isn't specializing might have, for example, a score of 2 in relevant Arts. His base casting total is hence merely around +5 [1 (Sta)+2 (Tech)+2 (Form)], enough to cast a level 20 spell in a supporting aura with fatigue given a bit of luck. Within his area of expertiese, he'll probably manage to squeeze through another magnitude for a level 25 spell. Very respectable too.
Spontanous magic will be at lower levels, of course. The specialist might cast a level 14 spell, the generalist a level 6 spell. Not overly impressive, perhaps, but enough to allow the generalist to do some minor tricks and the specialist to do something somewhat meaningful. I think that's enough for spontanous magic to still be significant, and capable of saving the day at the hands of an inventive player.
A major effect is that for young magi even small bonuses now become important. Investing confidence, burning raw vis, using a booming voice, the local aura... and of course the various sundry bonuses ("let's wait for the moon to rise, so I can cast the spell..."). The Mythic and colorful elements become very important, which is cool.
To climb from Art 5 to Art 10 requires 200 XP, which is probably about 20 Seasons (assuming around 10 XP per Season, e.g. by a level 10 well-authored Summa) - 5 years, probably spread out over a decade or more.
To climb from Art 0 to 2 requires 15 XP, i.e. it can be done in one season with a good book (a low-level Summa written by someone with a high score). The requirement to have a score of 2 in every Art to teach an apprentice is just like the requirement to have a score of 5 in the core rules (15 XP).