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.
With that being said though... I don't think Penn's jitsu is much better offensively than Diego's. Both rarely trow subs from bottom and on top position its mainly G&P or a simple RNC if the opponent gives them their back.
Defensively I admit Penn's guard is legendary and Im still mad at Sherk for not putting it at test. Sherk is pretty much known for his insane guard breaking skills (re: Florian fight for example) and the fact that he chose to stand with Penn pisses me off.
I agree we wont see Diego subbing Penn, but I do think Penn will get a run for his money standing, because Diego is more versatile and explosive than Penn. Im pretty sure Penn doesn't have the reach advantage either so we wont see him controlling Diego with his jab like he did against Sherk and Hughes (the above mentioned guys with low reach and bad stand up overall).
Should be good fight though, but no Diego wont be as easy fight as many Penn fans seem to think.
"When I land that left hook, the fight will be over." -Paul Daley (He called it!)