Froom's wattage has not been released so no-one can know what it is.
You're probably referring to the Antoine Vayer article where he estimated that Froome produced an average power output of 446 watts on the final climb to Ax 3 Domaines which would obviously point to the use of peds. Since Vayer used to work for Festina he no doubt has plenty of experience dealing with incredible statistics. 
The trouble with this kind of speculation is that it can be way off. There are too many variables - rider weight, bike weight, kit weight, rolling resistance, drag, wind resistance and direction, energy saved from drafting/wind cover... It's unlikely that even Sky have all the figures, which is why they are reluctant to put them in the hands of people who think they're experts because they did the etape or they came second in a club race.
The tour has arguably never been 'clean' in the strictest sense of the word but I believe it is at its best ever and Froome's dominance is not down to drugs. To explain it as such would have to mean that Sky were the only team doping which is palpably absurd.
