10-15% will have a much more massive impact than you suspect it to be. Imagine a warrior with full mythic, Pavise, and +5% durable passive + the 10-15% increase. Even with all the dmg in the world (I'm exaggerating over here), it'll be virtually near-impossible to kill that warrior. If anything, there should be little specific buffs/nerfs here and there for each particular class to even out things a bit more.