Pipe, Silencer, Reeds, Bolt Mod, ect all make a difference. It was put well by the k5Abuser, each part is an upgrade, gaining %'s with each part. All upgrades together make the monster. Don't know if you keep stock power curve, but you can adjust new power to be linear. I mean, as long as it is true and consistent power, it doesn't sling you off the back when it surprises you, because you only gained top or something.
My personal opinion is that reed spacers and Flywheel weights, only make the bike easier to ride, not giving you power gains. "Thats good too", I know.
500's are sweet...