I dunno 'bout that. You gotta admit that Barros and Roberts are simply excellent rain riders on any track, so I can't disagree with your first two choices.
Beyond that, however, you have to start considering things like who knows the track best and who's willing to take the biggest risks. I think those are two reasons why rain benefits the hometown boys the most.
So, I'd put McCoy up there, too, and maybe expect some unusually good results from Anthony West and Brendan Clarke or some of the other Aussies in the race... if it rains.