A word on slip on's. Of all the emissions standards set by the EPA, noise is the one most often overlooked. They measure all the noise the vehicle makes, exhaust, mechanical, tires, and if it is below the limit in force at that time, they certify the vehicle. Adding louder cans is an emissions violation, if they cause the vehicle to exceed the noise standard it was certified to. The EPA measures it as one total sound. The EU is worse. They measure it at idle, part throttle on the dyno and full throttle on the dyno and they set the sound pickups so as to zero in on the actual sources of noise. And they bring them back in and recheck them after the vehicle has been in service. One of the things lots of manufactures to move away from valve adjustment pucks.
One of the reasons for the M8 going to a single cam and fixed rocker shafts was to loose some mechanical noise. Mechanical noise is one of the hardest to meet standards in the Euro certs.