I'd be possible, but the percentage would really need to lowered then, since you'd effectively get (5+10+15+20+25) = 75% extra damage instead of 25%. So, if anything, it'd do (n+2n+3n+4n+5n)=25, n = 25/15 = 1.67 % additional damage per stack.
Truth be told, however, this is meant for bosses. It gives you a way to get back a decent amount of mana, and while doing that, you have to decide if you sacrifice for extra damage for mana, or keep going all the way for the large 25% burst at the end.
By changing it, the skill would be a lot more linear, and require much less thought. You'd take it as granted, and see your damage going up and down like a sine wave. I think it is more satisfying to see a large burst at the end, instead of seeing your damage climb slightly each cast, barely outside the random damage range (most skills do x-y damage) - and then, when it's decent see a large drop again.
Of course, you could say I shouldn't remove the stack once it reaches 25% - and maintain the bonus at 25% damage. However, that would completely OP mages. I'd need to lower their damage by 25% to get them in-line again, making this skill way too important (even mandatory, since you'd do 25% less damage) - and yet it'd do nothing else but bend the numbers right again.
I sort of try to avoid making passive skills that essentially don't do anything besides saying "hey if you take me, your damage automatically becomes good, and if you don't, it won't".
