Honestly, mitosis itself isn't the problem. If it weren't for SoR abuse (aka instosis), it would be perfectly fine, maybe even a little bit UP.
The thing that needs to be done is nerf the SoR - mitosis combo directly without nerfing either SoR or Mitosis individually.
Now that SoR has already been nerfed a bit by taking the spamability away... FINALLY! I'm not sure nerfing the mito-sor combo is as important... I do still like the idea of having giving it a chance to spawn a mutant or some other fun and unfortunate result if used with SoR. That would be quite amusing in its own right. But I don't think its essential now.