You should forget hermetic magic in this case. Supernatural virtues cover this concept far better. And goetia.
Yeah, I know. The trouble is, I like hermetic magic. It's fun and, with a few tweaks, it is flexible enough to cover all manner of "mythic" magic under one system, rather then a lot of micro-system. The "trouble" is the sheer amount of not "mythic" magic it also allows... and that, IMO, falls under the social contract of the game. We agree not to pull the kind of stupid crap mentioned in this thread (no offense intended to anyone).
Ahh... I see the disjoin. Rather than saying a magus should take a social status virtue, what I should be saying is a magus should take one of the following social status virtues: Clerk, Gentleman, Magister in Artibus, Priest or Wise One... that clarifies my intent much better. Thank you.
Do gentlemen work?
What I see here is that magi DO deal with mundanes in a way that weould blatantly fall into a breach of the code. Even if you pass over the quite obvious problem of the king of france pressuring the local bishops to give him more of those fire-throwing clerks for his next chevauchee, magi acting as court advisors and doing magical stuff for their patrons should be fairly common here. The level of interaction with mundanes is much higher than in the official setting...
So a Verditius swordsmith is out? Or a Bjornær huntsman. What about Redcaps gifted or otherwise?
I understand where your coming from and am not offended. But using magic to make money is really more clever then stupid. Exactly the sort of thing bonifide super geniuses would come up with so they don't have to deal with mundane crap like making money. The logic goes that if I the player with far less then a +4 intelligence can come up with half a dozen ways to use minor magic to cover expenses my character could come up with four score methods during breakfast.
A gentlemen's agreement not to use magic that way can work but it can also break down as a game goes on. For instance the first time needing money becomes part of the plot. Or when the table notices how much faster the characters who took Wealthy are advancing. Or if quite by accident a windfall lands in the players lap.
That's why I recommend external forces, like an even greater need for secrecy, as limiting factors. Although I'm not a fan of the peer pressure within the order reasoning. Are you going to piss off the guy who spends twice as much time as you studying, researching, adventuring and otherwise becoming a magical bad ass by saying, he doesn't know what real magic is for. Especially if he could brew you up the best Longevity Potion in the order or write that summa you really really need.
So... why exactly hasn't the king made some of these fire-throwing gentlemen landed knights, again?
You might want to completely axe learning magic (well, Arts, spells, and so on) from texts. Each magus has a unique Gift and understanding of magic - that's why one needs to invent spells instead of learning them, after all. And magic is a mystery that cannot be conveyed in words; learning a spell is a matter of self-transformation, not the accumulation of knowledge. Personal teaching still goes, and also raw vis - the scarcity of both methods should limit magical growth considerably.
Has anyone noticed that throwing lighting from your fingertips has a +4 magnitude modifier for being very unnatural, but hurling fire from your fingertips is a cosmetic effect?
Clearly, I'm just going to have to sit down and revise the entire spell list and probably many of the spell guidelines to better fit the vision of Mythic Europe my saga embraces...
At what point does it stop being house rules and become a completely different game "based on" Ars Magica? 
I would rather take YR7's system and customize if it needed. He made a lot of calculations already.
Your concept has an obvious weak point what I wrote earlier: what happens if someone will be able to free himself from working? The reason can be anything, charming a rich npc or creating a rich one for example. Are you able to investigate and eradicate every such issues?
It makes perfect sense to me. In setting Auram magic was based on weather control magic so it has artifacts of that style of magic, particularly the unnatural effect modifier. Meta-Game it balances out the legacy spells. Game balance Auram is way powerful so it needs something to hamstring it.
For all it's warts ArM5 is still one of the most consistent setting/system combinations ever. (I'd say I was brown nosing but it's not a very high bar to jump) In broad strokes almost all the major features of the setting, and most minor ones, inform details of the system and vice versa. You change one without changing the others and you can loss some of the cohesion that makes the game what it is. But I'm a dramatic simulationist so that crap is important to me.
The point where ArM stops being ArM is the point where the people your playing with are annoyed that your still calling it Ars Magica. Or maybe the opposite is true it stays Ars Magica as long as no one would let you get away with calling it something else.
Not necessarily. We have played "Ars magica" with homebrew systems before, keeping only the setting but not the rules at all. 
A very interesting variant.
Not sure I'll ever use it, but muse about it? certainly!
Changing guidelines so they suit your "feel" better is almost certainly THE BEST way to adjust power level, but beware though that it´s not exactly easy.
In comparison, in my last game PeCo Guideline Kill a person is Base 50 while some of the low end guidelines are the same as originally, most of all what the change i´ve mostly played with has done is stretching the distance between tiny effects and "uber effects" guidelines.
Another useful option is to make more fatigue from casting normal.
A suboption to that could be to change it so you have to devote part of the casting total to avoid fatigue.
So if for example you say that the normal is 3 Fatigue from casting and a magi needs to use 5 from the casting total to reduce by 1 Fatigue level, that means magi either gets less penetration or more fatigue.
If you want labwork to be less "easy" you can use a rule that ALL labwork uses up at least 1 Vis or the labtotal takes a penalty, like say it gets halved if you have no Vis.
This(or something very close to it) was discussed before here. Roughly speaking, Incantation of Lightning "wastes" a few magnitudes -- but this "stretching" seems necessary to simultaneously achieve all of the following:
a) maintain coherent and concise Auram guidelines
b) keep Incantation of Lightning at the same level as it was in previous editions
c) keep the damage coherent with that produced by a "realistic" lightning bolt
d) keep the level necessary to deal that damage with Auram at least as high as the level necessary with Ignem -- the "gold standard" for damaging spells.