Those findings aren't all that remarkable on their own. The purpose of the report was really to determine whether per capita profiling could be used to measure physician resource use, and the results suggest it can. And the entire point of figuring that out was to go one step further and provide physicians' feedback on their resource use.
Will that have any effect? Researchers asked private insurers who have tried similar programs that question and concluded that feedback has no more than a moderate influence on physician behavior. Moderate could mean many things, and the GAO seems to be hoping that feedback from CMS will be more of a motivator than feedback from a private payer "because of the relatively large share of physicians' practice revenues that Medicare typically represents."
Most physicians like data and enjoy receiving feedback on their own performance. Their competitive nature kicks on and they will often improve in areas they didn't even realize they were lagging behind, even resource use.
Even if the impact is only "moderate," that could add up to a lot of dollars.