Silva vs GSP cannot be a fair fight at catchweight because no matter how you handle the weight there will always be the question in peoples minds as to whether it was enough. Personally (though @lukethomas would disagree) I think the only way this fight makes sense is at 170, (or GSP at 185, but that's so unlikely i'm not going to address it here).

Heres why, this fight is for a subjective title "G.O.A.T.". Theres no G.O.A.T. award with clearly, and agreed upon specifications, and even if there were I doubt you could take into account enough variables to make it make sense. In the end you're fighting for public opinion not a quantifiable outcome.

With this in mind one of two things happens, if they meet at catchweight and Silva wins then he was picking on the smaller guy and it's in many peoples mind a forgone conclusion. If Silva loses it's because he cut so much weight and it just creates more G.O.A.T. debate instead of settling the matter. However, if you go to 170 and Silva loses then it's seen as a crazy attempt by an incredible fighter that didn't work out and it becomes, though a loss, a bright mark on Silva's legacy for having the guts to go down and meet GSP. If he wins GSP's camp is left without excuse and no one will be able to reasonably argue that he won by picking on a smaller guy.

No matter how you cut this the only way it works is if public opinion is left without reason to call foul, and I don't see any way you can do that at catchweight.