I've heard the upgraded touring seats with the backrests are hard to come by... a back rest is definitely happening though. I love the look of the Mustang seats especially next to the Russells, but I had a Corbin on a previous bike that I was none to impressed by. Might have to look more into them to see the difference between Mustang and Corbin.
The UP is really nice. great variance of scenery and best of all no one is up there. When you do see somebody it isn't an issue passing them. The only down side is that roads can be a mess cuz of the frost heave and plows beating them up over the winter. Crossing the Mackinac Bridge can be trip also. The winds can get whipping pretty good. Back in the 80's a Yugo got blown over the side. Also, most of the bridge is grating so you get that weird wobble on top of carrying a lean cuz of the wind. Its an interesting 5 miles.
Yamaha knows to refine a knockoff. I still have the stock electrical/charging system from 1977 in the ol XS. The only things I had to do was swap out the brushes and reset the voltage regulator. They're fun bikes to ride and super easy to work on. I considered selling it, and tried for a time, but I think i'm gonna end up keeping it. Throw a 750 kit on the top end, put some upgraded carbs on the back end, and have a cheap bike that's great in the twisties.