Not entirely sure what your definition of "proof" is but GSP's recent performances are hardly reminiscent of the career beat down's he used to dish out. Since the Serra fight he has positively changed. Against a guy like Diaz who is so mentally tough he has his work cut out thats real.
Gsp wouldnt even have to do damage to win this fight. It would just be a takedown fest, some top control- Gsp wins round after round.