Dawson would obviously outclass him, then most likely stop him in the mid-late rounds. I would say Diaconu has a better chance of winning than Johnson does though, because of his aggressive style. Neither of them have better than a 5% chance against him though. The current LHW division is weak and the only real threats for Dawson are Calzaghe/Kessler.
I've not seen much of Diaconu, but i think that if Dawsons somewhat average chin can hold up, and if he can keep focus, he'll win a wide UD.
Superheavyweight...Sorry if I hurt of feelings...but you know Dawson is going down and he knows also...I hope he had a nice trip to Mexico !!!