Have you considered the possibility that Valverde, Contador, Lance, Basso etc are simply better riders than the likes of Evans to begin with, prior to the dope? I think it's very possible. It could be that they are all doping equally and the best come out on top. It could be that there are gross inequalities in terms of doping programs. What seems certain, however, is there is no way in hell Sastre and Evans (in addition to the rest) accumulated the palmares they have dope free. If you look at their careers, neither has been on a team that could be considered clean, or where doping wasn't encouraged. I guess I'm jaded, but I've been following this sport a long time and there isn't a single Pro Tour rider that I believe is dope-free.
Really the ugliest aspect of the propaganda the cycling media encourages (ie that part of the peloton is clean, while the other is doping), is that fans naively assume the race winner is doping, while their favorite coming in at place 8 or whatever must be the best 'clean' rider. It's fandom based on the will-to-believe, or a kind of wish-fullfillment.