Um, Grievous Wounds isn't going to be doing a whole lot when the enemy has 100% lifesteal on top of AD-scaling healing from W and whatever lifesteal he might have from items. Think about it; a 40% decrease means he's still got 60% lifesteal from Cannibalism alone (and you've probably locked yourself into a subpar build/summoner spell setup). Even at Rank 1 it's 30% with Grievous Wounds applied. That's still a nightmare to deal with.
Then there's the AoE heal, which is separate from the lifesteal and won't be decreased unless everyone on the enemy team has Grievous Wounds applied to them. And even then, it's still healing them for 30% of damage dealt (effectively 30% lifesteal).
No, Cannibalism with those numbers cannot be balanced on Aatrox. You'd probably want something along the lines of 20-40% lifesteal and 10-20% team heal for 10 seconds if you want the ability to be balanced.