P4P is often argued, as in this case, of record vs record. Who did GSP beat and who did Silva beat? To me P4P's always been a different hypothetical... scale up GSP's body and frame to Silva's 6'2", and who would win if they fought each other?
I don't blame GSP for not wanting to move up in weight and fight Silva. GSP knows how easy it was for him to handle Penn in their second fight. Sometimes size... not just weight, cannot be overcome if skill is fairly equal. On top of Silva having about 4" in height on GSP, his proportions are different. The ratio of Silva's legs to torso are different (longer legs). Silva's an excellent striker with kicks, and even if they fought each other at the same weight, Silva's reach advantage with legs and arms would offer a significant problem for GSP.
But if I could play God and expand GSP's height and weight to match Silva's... which of the two would win now that skill, fitness and determination are the only differing factors. Kind of an aside, but this is why I think Jon Jones is going to rule the LHW division the way Silva and GSP have ruled their classes. Jones' frame, height, leg length and size are almost unfair for the division.