The closest that I have come to a Sportster was my '93 Vulcan 750. Great bike, but in spite of a liquid cooled, counterbalanced engine, and the Mustang seat that I added, my saddle time was about 45 minutes without getting off to stretch. Obviously vibrations from the engine weren't the problem, but you could feel every little bump and dip in the road. I attributed that to the bikes 500 pound curb weight. Moving to the '99 Ultra solved that problem. Nothing wrong with trying different things to make your ride better, but IMO the solution for a lot of the problems I encountered with the smaller bike was just the addition of a lot more weight beneath you.