I think he would be able to compete. However, I don't think he would be able to compete with the top tier guys in the division, without some adjustments to his game and or physique.
I could see him losing to Frankie Edgar, Tyson Griffin, Ken Flo, and probably even a Nate Diaz. Maybe some others I could list but I just don't think he matches up well with most GOOD UFC fighters.
I think wrestlers would dictate the fight keep it standing and his striking would be exposed. Or he'd fight a great standup guy with decent grappling to survive on the ground, and then Aoki would still get beaten on his feet.
Also, putting him anywhere near BJ Penn, even in the same sentence is kind of laughable imo, we all know BJ has better grappling credentials and is an amazing athelete. Aoki's striking is lightyears behind BJ's, who is arguably the best pure boxer in MMA.