Ultra, I recently put a HD Sundowner on mine. \\; I'm 6' and 300+, and needed something with a wider seat than the stocker. \\; Also have the HD adjustable back rest. \\; So far, I'm satisfied with the seat. \\; For me, it's more comfortable than the Mustang I had on my previous bike, and was about $200 cheaper.
Have your stock seat custom done. The Sundowner looks to big and bulky on a SG. You can find a local sticther in most places. I went to www.bitchnstitchninc.com in Colorado. They did my seat for $225 with gel.
I'm 6'6", 280 and I just went with the tallboy seat on my SG. I have done a few 300 mile days with no problem, but 400 is about the limit. I think I am going to get gel inserts this winter from mean city cycles, read lots of good things about them.