I attended a seminar by Roy LoPresti. He was a major factor in the redesign (for speed and efficiency) of the mooney aircraft (J model) and has been involved in numerous speed mods for different.
He stated that their research had shown that the advantage of a clean airplane over a dirty one was negligible. Same with bugs on the leading edges. Yes, it surprised me, too.
In my personal experience, I have never been able to notice any difference in fuel consumption (or performance) whether the plane was dirty or clean.
YMMV, of course.
