I think that's a neat idea to produce more limited magic, which puts the magus in greater focus and risk. It would kill weather-magic and limit discreet-casting, as noted, which I think works well for a low-fantasy saga.
I'm not quite sure what you mean by low-fantasy, but the key limitation here seems to be simply Ars Magica's powerful magic. I'd recommend doubling all spell levels and lab totals, at least; I play by these rules, and the magic is still quite powerful (not quite "low-fantasy").
I also double or triple Magic Resistance, and have critter's main attack have penetration equal to its MR (if their main attack is magical). I think this is quite necessary for a low-fantasy setting, to make the stories more about using wits or arcane connections or just martial might to defeat the fore rather than brute penetrating the creature's magi resistance.
I think Ars Magica does low fantasy well, if your're focused on Mythic storytelling using fey bargains, raw vis harvesting and use (including enhanced vis), arcane connections, and so on.
If you mean to use mundane combat a lot, you may want to improve it - I find ArM5 combats can take far too long - although I don't use the Groups options too well. I think you can easily add +5 to damage across the board to speed-up combat and make it more frightening.