On speaking purely in the UFC, I certainly think that the P4P rankings certainly have been shaken up a bit, indicative of this thread. They currently have three champions at the top of their divisions and then everyone else in those divisions, Penn, GSP and Silva. I'm leaving Machida out of the conversation for now because although I don't think it was as controversial as many believe, I still had Rua winning their fight 48-47. That being said, I agree that GSP is a beast I'm not sure if he would have success against some of the top MW's. AS has already shown that he can move up and dominate. How would he fair against Rashad, Rampage, Rua, Machida is obviously up for debate, but going on facts, he would do just fine. BJ Penn has been a WW champion, beating Hughes and although he lost the other three times outside of his weight class, he has consistently shown that no one is in his world at LW. And against arguably one of the most aggressive punchers in UFC, he got hit a total of 8 times. I think that Silva is similar in that regard but I don't think GSP is.
P4P= 1. AS, 1a. Penn and 2. GSP