Couldn't find this anywhere by searching and it works aswell on Normal Cho'Gall
If your corruption is at 90%~ or so, save your Shapeshift cooldown (Iron Boot Flask, Metaporhasis, Savory Deviate Delight) and use it just before you hit 100%, youll get a 100% damage increase for the full duration of t he shapesiftand afterwards you can still recieve healing, despite being at 100% Corruption
Hope this helps (:
(Only Tested properly as a Demonology Lock with Metamorphasis Cooldown, however this may be a general bug with "Transformations")
Don't Forget to link here for Reposts, seems someone else is trying to Mooch it already.