Pretty sure it is 30%. Keep in mind that it does take a lot of health... makes it so it's not really worth using. It also costs a bit of mana to turn on (around 40) and 30% M.Atk. is not 30% more damage, it's more like around 15% more damage. On top of that, it makes things cost 10% more mana as well.
The m.atk increase is 30% which is pretty decent, but it does drain alot of hp and have the increased mp cost too. Its really good for SH and necro who can just use drain to get their hp back.
It seems like it was designed to be used either in combination with CDL or Dark Vortex (for Necros and SH). I get >1000 HP back from each shot of Dark Vortex. It is a long way to 78 for me but the one skill I've been looking forward to the most (after I get Wind Vortex) is Arcane Power.
Your damage isn't linearly proportional to your matk. IYour damage goes up as the square of your matk. In other words, if your matk goes up by x amount, your damage goes up sqrt(x). So in your example, when you doubled your matk, your damage would go up by 800 * sqrt(2) = 1128.
It seems like it was designed to be used either in combination with CDL or Dark Vortex (for Necros and SH). I get >1000 HP back from each shot of Dark Vortex. It is a long way to 78 for me but the one skill I've been looking forward to the most (after I get Wind Vortex) is Arcane Power.
It seems like it might work good until you realize all the mana you spent draining to get your health back could have been used with your main spells and you would have done more damage per mana point. The only time you really want to do tons of damage and not worry about mana is in a PvP, and that health drain isn't very nice, because you can't drain CP, and if someone is hitting you really hard... you don't need to be helping them hurt you.
Anyway, everyone is pretty much right. For a Spellhowler or Necro, it is an ok skill, although it is nothing compared to some of the other 76+ skills. Since the person who posted is a Sorcerer, I would have to say it is probably the most useless active 76+ skill you get, right behind arcane chaos. I've tried this skill in PvP and PvE and the only time I ever found it useful was fighting a Prophet in the Olympiad... which I lose to every time anyway, so there was no big change there. As far as how fast it takes your health... when the Prophet finished draining my mana I was about at 1/3 health, and they had not hit me a single time.
For olympiad I loved this skill on my PTS overlord. 30% more m.atk makes a huge difference on debuffs and is definitely worth the 100-200 hp total you lose during the fight.