I'll give you my subjective, seat of the pants (literally) answer on the RSS.
I own 2 Buells - a 04 XB12R and a 07 XB12X. They're almost identical in the drivetrain - 1203's, stock mapping, drummer exhausts.
When I rebuilt the Firebolt (stupid drunk driver) I decided to do some heat reduction mods while I had the engine rotated. I insulated the frame using the adhesive shielding sold by ASB. I also wrapped the pipes, as they had a ding in them from the accident anyways. I added the RSS a couple months later. I can't say if it was cooler than stock form, as I bought the bike from a friend after it had been wrecked.
The XB12X is stock - no RSS, rider comfort kit, frame insulation, or header wrap. There is a noticeable difference in heat between the 2 bikes. The mods on the bolt make a difference, as least from what I can feel when riding. Both bikes are okay when moving, but I hear the fan running more on the Uly, and it definitely feels warmer when I come to a stop.
I'd love to get one of those laser thermometers and see what the temperature is on the right side of the frame of both bikes after a few miles... if one of my friends visits, maybe we'll do that and see if there's a significant difference. I'd also like to compare that temp to the Uly after almost the same mods as the Bolt. This fall I plan on insulating the frame and adding the rider comfort kit, but I will not be wrapping the pipes as the Uly's header is perfect.