The problem with the "keep it stock" theory, is that due to their ancient design, to get a Harley thru EPA testing, they have to literally strangle it with super-lean carbs (or EFI) and super-quiet mufflers.
Just by replacing the stock carb on my XLH1200 with a Mikuni HSR42, replacing the air filter with a K&N, and replacing the OEM mufflers with slightly louder (but not as loud as the pipe on my Honda VFR750) I've probably increased the BHP and torque output by more than 25%.
While loud pipes do save lives, on a Harley, along with a better carburettor, they're pretty much a necessity. Cheers, Terry.