Penn's standup is WAY overrated. He hasn't really fought any high caliber strikers so far. He outboxed Sherk and Hughes mainly because both of them got pretty terrible reach and standup overall. Florian was winning the standup against him (not by much, but he was leading on points even though Penn wasn't getting hurt alot), GSP wrecked him standing & on ground, Pulver pretty much tooled him (on first fight, not much standup at all on second one) and many (including myself) thought Serra got screwed over by that decision against Penn too.
Yes his standup is solid, but he doesn't trow kicks and isn't as explosive as Diego is. Diego brings a whole new challenge to Penn as he can really test Penn standing and has good enough ground game and TDD to keep the fight standing unlike many who have become victims of Penn's jitsu.
Yeah but GSP was really smart about it. He wrestled and clinched BJ for the first round to get blood in his shoulders and that limited his mobility and slowed down his boxing, that's why he recked him so bad. By the way, I didn't spot this myself I'm really not that smart to do that, I watched the fight again recently and he said it post fight.
HW: Junior Dos Santos
LHW: Gegard Mousasi
MW: Anderson Silva
WW: Georges St. Pierre
LW: Kenny Florian