I can not use this cuz I have an R.
If you want to sell, just post and it won't take long.

 

Mustang Seat

Might be a C thing because when I briefly had my passenger seat on my R there was no issue whatsoever.
You might want to try it for a few hundred miles though to see if you get used to each other.

 

for what its worth...give it some time first to break in.
I didn't like my mustang seat at first either, but after a few rides I kinda settled into it.
Now I wouldn't trade it for the world.

 

Wouldn't trade mine either, but can't speak to the Ultimate. You'll get used to it, it is designed to do exactly what it does. Because the Mustang is "low-slung" on a C, the passenger seat is designed to act as lower back support. Gets quite comfortable after you get used to it.

 

The Mustang passenger stacks on top of the riders seat forming some what of a backrest like you said. The Ultimate passenger sits a bit farther back making room for the optinal add-on backrest.
I rode the Mustang set for 25,000 miles and went to the Ultimate, and now after another 15,000 miles I find no compairison in comfort between the two. Ultimate! hands down. :wink:

OBTW,
Both seats sit the same for forward and back but the Ultimate (due to the extra padding) does sit you a bit taller, say about an inch.