This article gives an idea about the power profile that occurs in pro stage races...
http://theconversation.edu.au/how-did-amateurs-compare-with-pros-at-the-tour-down-under-11946
When I was a grad student at the AIS, I shared an office with another student who did lots of these analyses on stage races (in particular the TdU). Pauwel's power profile during that stage is typical. Pros spend large chunks of time rolling about at power outputs that most good amateurs could handle, but the difference appears at strategic points in the race eg: breakaways, towards the finish, mtn climbs and ITTs. During these critical moments in the race, pro level riders produce power that is in excess of what the best amateurs can achieve.
IMO winning the tour is about 4 things, 1. having the engine to produce the necessary watts/kg or watts/CdA to put in time gaps (or loose the least amount of time) against your rivals in mtn climbs and TTs respectively, 2. having the endurance base to arrive at the bottom of the climb feeling relatively fresh 3. having the team support to deliver you to the climb having expended as little energy as possible and protecting you on flat stages 4. being able to recover effectively over consecutive days.
If you don't have the first 3 sorted, then #4 becomes far more important.