Let’s recalibrate Fifth Edition!
ArM5 is designed with some rough balance in mind - between virtues, between the power of magi and their place in the setting, and so on. However, I would contend that in practice this balancing act went somewhat awry. So I’m suggesting here to recalibrate some of the rules and numbers. This is not a redesign, only a recalibration to make things somewhat more consistent.
So - here is my suggestion on calibrating, first, what power Art scores magi can reach, and a few derived results. Do tell me why my calibration, too, is off. 
Chapter 01 - The Foundation is Set by the Limits
I want to start with the expected Art scores, as I think these set the scale for everything else. I will begin with the standard ArM5 progress, slightly modified:
The stop-measure is there to force one to spread his expenditure. In-game, this would usually be because the character has other interests that push him to invest in other Arts, invent spells, go on adventures, master spells, invest items, and so on. The rate of 30 XP per year, in turn, corresponds to roughly Sources of quality 10, with one-quarter of seasons “wasted”.
Low-level summa would have higher qualities, so that at high levels typical qualities should be a bit below the standard to maintain the 10 as “average”. This is a very minor effect, however, over a magus' long career.
We shall also keep the Warping progress of ArM5, again with slight changes:
The accumulation is from 1 pawn per year due to the longevity ritual, plus an average of 1 pawn per year from magical botches and other events. Player magi can deviate significantly from this guideline, but it is assumed NPC magi generally do not - unless the storyguide wishes otherwise. Even at old age, magi will not drop their longevity rituals and will continue to accumulate warping due to other reasons.
Final twilight at Warping 10 is applied to derive an effective maximum on Art scores. Without it, they are unlimited. A Warping Score of 10 requires 275 Warping Points, which is 137.5 years. This leads to the effective age restriction on magi,
Older magi are possible, especially through Mysteries, but should not affect the setting’s power level.
A related issue is Twilight. If a magus can spend decades in Temporary Twilight, that can increase his age. But it should not grant him more XP than a corresponding amount of Warping gained by regular years would have. So our cardinal rule on Twilight is
This means we can effectively ignore Wizard’s Twilight when determining how it affects the limits. Player characters may still gain XP (and Warping!) quicker than the standard 'warping at later life' guideline by undergoing Twilight, but we shall assume that it all balances out for NPCs, so they still just gain 2 WP and 30 XP per year, Twilight included.
We can now determine the highest Art scores in the Order. Without taking Magical Affinity into account, 140 years at 10 XP per year will grant 1400 XP, for an Art score of 52. Magical Affinity is too hard to let go of, but in order to keep things somewhat sane let us limit it to +¼ of the XP; this leads to 1750 XP, or score 58. A score of 60 would be 1830 XP, which is a bit further but not impossible to gain surely.
We can use that as the absolute maximum level the Order has developed so far.
Specific Arts perhaps never had a magus that reached such high scores in them, nor is it likely that the grand masters who penned the highest-level summas also had good communication or the Good Teacher virtue. Nevertheless, good, high-quality summas can probably be found at levels approaching this limit for all Arts.
For a sanity-check, consider a Quality 15 (6 base+5 Com+4 Good Teacher) summa at level 30. This is 465 XP, so can be read in 31 Seasons, supposedly over 31 years “on average”. The magus would still have some 109 years left, which with “standard” quality sources will bring us to 1555 XP which is a score of 55. Adding a ¼ affinity we reach 1943 XP which is still within an Art score of 61. So a maximum Art score of 60 still seems rather reasonable.
All these guidelines are there for NPCs and the setting. PCs can break them. In particular, they might be able and want to gain more than 10 XP per year in certain years or even on average, thus growing faster than the guidelines suggest; and they might be able to avoid gaining Warping points to expected level, and so on. However, these guidelines now allow us the foundation to build the rest of the system in their light.
Changing the rules.