Google wouldn't take the risk loosing credibility when doing that. It would certainly come out because SEO gurus will test similar sites with and without adsense and publish the results.
It would not make sense in the long term. They would lose trust and profits in the long term. In the short term yes they would make substantial profits if they did this but lose them in the long term.
it certainly would make sense for google to do that but i don't think they do. their main objective is to give the best quality search results regardless of whether ads are on the page or not.