Has anyone worked out the math on how malignant cells would work with this?
It seems to me that if you have a field full of malignant cells (not hard to do) and half of them are growing each turn it could get ridiculous pretty fast. Yes, a lot of cells would die, but they'd grow back the next turn. Heck if you had a bonewall that'd be helpful. Some Chaos Seed effects could be helpful as well when cast on your own guys (PU, Drain Life).
I'd suggest altering it to a random stat change that could be either positive or negative (better chance for positive though, still want it to be useful). That way malignant cells would be more likely to be killed by the mutagen, while it wouldn't be weakened when used with abominations.
Just a thought, take it or leave it as you like.