I just wanted to add to this, from another quasi-scientific point of view.
Should you have said loud pipes in an area that is conducive to sound waves being reflected back towards all vehicles (think an amphitheater here, like downtown in a city with high rises, etc.), than what is taking place is that some sound is being reflected back towards vehicles, allowing for motorists to potentially become aware of a loud noise (a.k.a. your motorcycle) somewhere around the vehicle (which admittedly, *may* save lives; however, we all assume that a user would react in a logical fashion and check blind spots, etc., which we know doesn't always happen).
In a scenario where there is no ability to reflect sound back towards a road where vehicles travel (e.g. countryside, open highway, etc.), it may be more difficult to prove that someone actually would hear a bike moving up through a blind spot.
Again, I know of no hard proof that either supports or debunks this view, but again, everyone is entitled to their opinion on the matter given the nature of the "facts". A search on the 'Net yields nothing conclusive, however, I found this article here:
http://motorcycle-intelligence.com/do-loud-pipes-save-lives/1119/ has the best advice (echoing several posters in this thread):
"Even though scientifically unproven, if riders’ lives have been saved because of loud pipes, or if riders believe they are safer with loud pipes, those are some compelling factors – at least for those individuals."