It stretches and snaps back like a rubber band robbing power off the line and each time you shift. There is a huge difference dynamically between a final motorcycle drive a driving a supercharger or timing but in timing performance wise chain is better than a belt and even better yet is gear to gear, each step reducing lash. I'm not going to continue to argue this since I really don't care what you believe or run, have fun with your sportster and belt drive especially when it comes time to change it or it fails during a ride and you need a trailer. By the way most of those sport bikes you mention also run chains. Belts like the cush drive are intended to absorb shock and vibration thus giving a smoother ride. Cush drive would rob some power just as the belt does, I like the way my old bikes sprockets bolts right to the hub, no rubber parts.
Robbing power is probably the wrong way to put it as a belt would not effect HP over all it just takes some of the snap out of the take off due to the stretch, then it comes back that being different from the way a chain stretches over time from use. A chain stretches needing adjustment from time to time until worn out a belt just eventually fails.
Won't strip the teeth off a chain drive like you will with a belt under extreme use, if you are stupid enough to use aluminum sprockets then they wear out fast with a chain, not sure with a belt.
