The GSP who tore down Serra would eat Rory for lunch - the GSP you saw against Fitch was GSP Phase Three.
Phase 1: Debut to Matt Hughes - very confident, tears it down, bright young buck. Hughes humbled him in a classic and that fight evened out GSP bringing him into his prime.
Phase 2: Prime warrior who rocked out Sherk, Mayhem, Trigg, B.J. Penn (prime). He was the first real rock star of MMA, not Tito Ortiz. The "just bleed" eminem fans weren't into MMA yet, and GSP was using his "wise man/karate kid", polite/humble/respectful gimmick. This GSP ended when he was shocked-and-rocked by Matt Serra, who did the unthinkable. Serra taught GSP that he wasn't invincible, but GSP ate his soul in the rematch and Serra was never the same guy again.
Phase 3: To defeat middleweights who're playing the weight cut anorexia game, he needed a better plan and couldn't gas. He needed to fight a patient fight, playing a game of Buehler Blue - that is, a game of stalling, countering, and high striking volume but minimal energy investment. This is the GSP who you're calling prime - I believe Phase 2 GSP and Phase 3 GSP would both beat Rory because GSP is really patient and Rory walks into things and gets hit with stupid stuff. Rory also gets flustered easily when things aren't going his way. This plays into GSP's game.