I think the whole deal behind sneak crits is that the enemy doesn't have time to move a little and cause your aim to be off so you can hit precisely where you're aiming so you do more dmg because you hit directly into the heart or whatever and you could aim for slits in armor or critical spots if they don't know you're there whereas if they do they're jumping around and blocking and dodging so you can't really aim for a critical spot, you hav e to aim more for an area. With magic i don't think it'd make a difference a flaming fireball is a flaming fireball, can't really aim for a critical spot with it, only thing sneaking should allow is a first shot when he's not moving so it's almost garuanteed to hit unless he moves at the last moment for bends over to pick up a coin
. now if they up the magic and have some spells like a spike of magic energy that stabs into the area you're aiming at then a crit could apply to that i guess but if they have that there's not really a point in being an archer or having a spear or something. So if they stick with the standard, boring fire, ice, lightning magic then no, there shouldn't be crits, but if they make something that crits could actually apply to then sure. I just don't see sneaking having an effect on lightning, a fireball, or an iceball besides being able to hit a little easier.