I wouldn't say Pettis is p4p king. I think he does enough to compete with henderson and get the win somehow, he isn't dominant.
P4P is obviously very subjective but you can't put Henderson in that discussion and not Pettis. Stylistically they are very similar. He doesn't do "enough" to compete with Benson? He damn near ko'd him the first fight and submitted him inside the 1st round last night.

I just don't see any elite losing to a Guida caliber.
I didn't see GSP losing to Serra but I don't think that loss somehow diminishes his career as a whole. Pettis needs to start adding top names to his win column to be truly be in the P4P talks but his talent level is there by a long shot. The Guida fight showed a deficiency in TDD but it looked like he has improved that area from last night.

He could of dominated that fight.
A lot of things could have happened, but what did happen was that he didn't dominate it and got submitted. He is the first UFC Champion (not interim) to lose their belt by submission in a decade. That doesn't happen often b/c it takes a very talented fighter to submit another elite fighter.