Cut down the stock Suzuki seat foam (with stock seat base) on my GS550, to about 2 inches max thickness in the forward area. Rode over 17,000 km. in 3 months on it. Felt fine.
Using 1/2 inch closed cell racer seat foam on my V-twin cafe bike. Feels okay, wouldn't do coast to coast with it though. For some reason, I like it better than the 1 inch foam I had on the previous cafe seat.
All my dirttrack / streettracker seats used 2 inch foam of various densities, worked okay. I think foam density is at least as important as thickness. You might also consider buying a gel cell seat kit, but I haven't tried one...
Hope this helps.
Regards,
Pat Cowan,
Pacomotorstuff