"The louder it is the faster it is" doesn´t apply especially for a Harley or some japanese custom bikes. The problem lies in their weird ignition pattern. Open exhausts with these bikes sounds horrible imo. For me a good exhaust sound should be deep and even, Ducati, RC51, Yamaha TRX 850 are good examples. It doesn´t have to be loud just a deep bassy even sound that also warnes other traffic about your precense. As for an in-line four cylinder sports bike, the sound of a 4 to 1 race system can be very annoying even to me.