Slowing down the surface seems to enable the better players to win more proportionally. With hard, it seems like there were huge upsets all the time. I'm not really noticing them since paying attention to the clay odds. Superior players go down like 1-4 occassionally or 0-3 and then win the match without a hitch. On hard, the inferior players can cheap point hold their way to upsets or bash the ball.
Feliciano Lopez and Big Dick Raonic are somewhat exceptions. I'm somewhat surprised that Lopez and Raonic are still winning a lot on their serve, but I guess it makes sense. If your serve is so powerful, even a slower surface is still not gonna completely nullify your game. They haven't pulled off any huge upsets. Gulbis just lost today easily, so he's just off. Ferrer killed both. Just a few small underdog wins.