So I've meant to post for about a week now, but I keep being busy.
In short, I am of almost the same opinion as @Euphemism.
Unless the player and NPC magi are going to go head to head in direct confrontation, there is just no real need to stat them fully. I ran a year long saga and stated two NPC magi, and even then only because they were to become the (Tytallus) antagonists of the players, fighting them in certamen, disputatio and in the final session, directly.
For my game notes, I had a big spread sheet with all the mages of the Rhine and Lotharingia, they approximate ages and their top 2-4 arts (depending on if they go tall or wide). Along with a few other notes, such as important virtue, personality traits etc (see below)
NPC magi which were due to be helpful, don't need proper stats, since they just grant some kind of benefit, even if it requires something to make up for it.
For example, Garrinchus ex Misc, in Cologne is a Co healer mage. I chose to have him as a safety net for the party when they started off in the Rhine Gorge. So when one mage would get beat up, they could just go there and use the room with the Room range CrCo bonus to recovery roll effect. All I had to do was chose what the bonus was and then that was it. No need for a lab total, Pen total (the party was just told to lower their parma if they want to benefit from it).
Other neutral NPCs could have tractatus, spell scrolls or spells as appropriate to their specialties and could cast based on their rough age... For the Verditius exhibiting at Tribunals, I tried to keep a rough ballpark, but it is more valuable for them to spend a season cranking out 3-5 low level items which will all find purchasers than one really complicated one which would have more trouble finding a buyer.
Examples:
Peter von Wurzburg (He)
The horseless cart (9 pawns Vis + 2 pounds silver)
A well-made cart carved out of a single huge tree trunk, with space for a horse, ox or donkey in front. It moves as normal cart would, but without the need for animals. There is a branch in a strange cross shape, used to control the cart. (Wilhelm Weiss has one such, but he uses his Im to appear to have a nag pulling it, Peter von Wurzburg has the first model and has a donkey attached to it, though it is only for appearances)
Level 10, +2 Concentration, +1 Touch = level 25 Item maintain concentration +5 = 30
Rhesus of Florum (An)
The lordly shirt (3 pawns Vis + 3 pounds silver)
A beautifully made shirt, it is made out of the absolutely finest silks and would not look out of place on the highest nobles. Two main colours are available: Aquamarine blue or Imperial Purple.
Aura of Ennobled presence, level 10, 1/day (ArM, p145)
Only if the players had wanted to have a custom design would i even have to think about what their lab totals might be. If the mage is of a comparable age to the party, then they'd have their top lab total equal to one of the magi's top lab total and so on. While the very old mages have much higher scores, the party just won't have the cash to pay Murion or Stentorius to craft something for them...