I like this.
I'd still use the errata, though. These are errors in the rules, not "old ways" of magic.
Something else I'd do is give Elder Magi (In Ars Magica, Archmage is just a title, after all) a lot of spells, and mastery to boot. After a while, even if you don't keep ubreast of the latest developments of hermetic magics, you'll invest in spells, both because it's more time-efficient than raising your arts, and because it's safer than spont. Likewise, mastery protects from botches.
Or at least this is the way I'm trying to aim lately when developping experienced magi. This gives them skill to go with RAW power.
Something else I'd expect from some of them is non-standard spells.
Not spells that defy the rules per se, although a given magus may very well has had his own breakthrough, but spells using non-efficient ranges and durations to achieve specific purposes.
For exemple, without taking the time to develop and integrate the range, one could have a spell with "Reach" range, covering anything that is could be touched by an extanded arm, akin to Touch + 1 for non-standard = +2 magnitudes.
Likewise, a "Room-Sized" spell, affecting a volume roughly equal to a standard room (Magnitude +2, +1 for non-standard = +3).
=> Even without special virtues, an "Archmage" might very well surprise younger magi.
Not only is he powerful (Good scores in the arts), he is skilled (A lot of spells, covering a wide range of possibilities) and can do strange things (Casting spell with non standard ranges, durations and targets).