I have been following a sports science website, called The Science of Sport, for a while now & the stuff on the tour is fascinating. Here is a excerpt written by Ross Tucker:
"...doping has an effect on the repeatability of the performance, just as much as it affects performance acutely. Many will think only of the acute doping effect, but in fact, most of the doping products exert an even bigger effect on recovery, and hence the ability to produce this level of performance over and over. Think testosterone, growth hormone, cortisone, insulin - all are used to reduce stress response or improve post-exercise recovery. Even EPO would have this effect. Therefore, one cannot infer too much from a once-off performance. Rather, you have to look at a collection of performances, which also partly addresses variability provided by wind speed, temperatures and race situation."
When you consider performances & wonder whether they may be drug induced remember that recovery is the performance key. In the tour obviously it's a multi-stage race, but in most other endurance events the drugs help the cheats by allowing them to recover more quickly & thus train harder & sooner & more - so all 3 factors: volume. intensity & frequency are raised & optimal performances are possible sooner & more frequently.