I'm happy with the 16" wide, however it is a fair amount of work to make it fit. Of course once the tunnel mods are done for the 16" a 15" will fit back in no problem.
I ride 90% mountains though, and to be honest if I was doing the swap for a couple trips out west. I probably would stay with the 15" wide just because it's easier!
A 2" is better for the powder then a 1.75", however if the snow is a little more set-up.(spring snow) You probably wont notice a huge difference between the two.
I'm not familiar with the back country lug pattern, but $250 for a new track is a pretty good deal! If the pattern is similiar to a challenger, I probably would go with it and your case.
If you decide that you like riding the mountains, and to make more trips out west. You can always sell the 1.75 BC latter probably for close to what you have in to it. Then "upgrade" to a 2" 15"or even a 16" for next year!
I highly recommend some mountain ski bottoms, it's an easy swap and helps big time with front end floatation/side hilling!
Pulling up the limiter straps on a XTX also helps deep snow performance, by reducing the approach angle.