Originally Posted by
DirewolfX
No, 20% damage for 20% more mana would be on all the time. It doesn't hurt your mana efficiency but it does increase your DPS. It's simple math.
Let's say you have 12,000 mana and can do 1,000 DPS at a cost off 100 mana per second. You can last for 120 seconds and do 120,000 damage.
Now toggle on permanent Arcane Power. You now do 1,200 DPS at a cost of 120 mana per second. You can only last for 100 seconds, but you still do 120,000 damage.
So same amount of damage, but done in a shorter period of time. On a long fight it doesn't matter; you do 120,000 damage regardless. On a short fight, you're going to do more with the magic AP switch turned on.