Penn will have his hands full with this fight. Sure Diego was "brawling" in his last fight at times, but that was only because he landed the first shot and was looking to end the fight. You can't fault him for going for the kill, or call him sloppy.
His striking was cleaner in that fight than it has ever been, and after a feverish 1st rd. he settled in a picked guida apart.
Penn is at a different level than most fighters at 155 but he is not unbeatable. What Diego offers that no other fighter has at 155 right now is a very well rounded game and he excels in all areas. He is an excellent wrestler, has ko power, can ground and pound, good striking, and good submissions. He can end the fight from anywhere much like Penn can, just not at that level. The key factor to Diego's strengths is his explosiveness. Much like Penn can throw his flying knee, Diego can mount, throw a kick, land a big punch. It will be hard to hit penn, but I doubt Diego will be looking to simply wear Penn down like Florian.
Diego-Penn is very interesting, and far from a lock for Penn like some guys are calling it. Diego is a solid, well rounded, explosive fighter. He hands down, has the best chance of beating Penn right now, and I personally think he will.