I was doing some testing with power attack while hidden and I noticed that the bonus damage from the skill wasn't always +5, it would show a +5 but the actual increase changed with different attacks. Sometimes it'd be +3, +4, +5, +6. I.e. the die roll would be two 2's (as I'm dual wielding, a whip main hand and dagger offhand), first attack would produce 6 damage, and second would produce 7. I also noticed that it was giving a -3 to hit rather than the -5 the skill states.