Well, nobody ever mentioned anything about Silva "downsizing". The fight being discussed would be at MW. So Silva would be in no different shape than for any other fight. GSP would be the one moving up in weight, where he would most certainly be in a more disadvantageous position than he has ever been at WW. That's just logical.
I certainly think it's fair to say that GSP has a fair amount of panic in his mind when things don't go as planned in his fights. Anyone who saw his discussion in his corner with Greg Jackson during the Sheilds fight or the way he blindly fled in the Serra fight could see it.
Like I said, if Silva had that same weakness, he wouldn't have had the ability to control himself long enough to finish Sonnen, and he would not be champ right now.
That kind of weakness can cost you a fight, and it did once for GSP (luckily not twice).