The concept is cool but it is pretty irrelevant because it is inactive for a large portion of your more important mana use area. Plus the 10% regen is a bit of a joke.
It should be always active, like dmg drops at 60% and regen picks up or something.
40% damage 60% regen. and the regen should be buffed to like 30% or something.
I agree it should make Mages never run out of Mana if they Single Target at a loss of some dps since you lose some good runes.
Actually I was thinking about it more and it would be cool if it worked in a way that lets you regen higher than its switch over state.
For example, you have 10% damage until you reach 40% mana then you gain regen, and the regen stays until you have 80% mana at which time it switches back to damage.
It should provide regen while casting or bonus regen if you are outside the mp5 rule. So you can get regen while casting or you can stop dps for a bit to try and get your damage buff back.