Does anyone have the Corbin Solo and still use the stock passenger pillion?
What "extra part?" It comes as one piece, you have to undo two bolts to make it a solo! The standard Corbin does move you back about 1.5" which was perfect for me, since I'm 6'1" and appreciate the extra legroom. I am pretty sure they have a photo with a slider that allows you to go back and forth showing the difference.
And I'm sure you've read about it, but Corbin's do take a while to break in - in my case it was about 1,500 miles for both of mine. It's not a big deal, you just have to stick with it and it will be comfort for life then. Some say it takes less time, some say more, so your mileage may vary. Literally...