Yes, the sundowner will work without removing your rails. I have one of the loaner models on Ginger right now and no rail issues....went right on with no issues. I suggest if you are going with the sundowner, utilize the seat demo deal at your local dealer. You can borrow whatever seat you are thinking of and use it for a bit so you and your BSR can see how it works for you. You don't have to buy a seat first and then have regret. It's a great program if you are considering an HD seat.
I've looked at Corbin (again) and considered the Mustang touring models but after a few miles with the loaner Sundowner, I think that's the way I'm going, too. Like the seating position for me and more room/more comfortable for my better half.
Good luck in whatever you decide.
Howard