I have a Sundowner seat I bought for my 08 SG that fits my 15 RG. I also had a 12SG seat that I used on my 08. Pretty much any touring seat from 08 up will fit.
I asked my dealer last summer about using an aftermarket seat from a pre-Rushmore model. He said it will not mount properly, but I don't recall the exact reason. That could be the dealer just blowing smoke up my rear end though to sell one of theirs. I'm sure someone else will have a better answer for you.
I have a Corbin and a CC seat that I purchased for my '09 Street Glide. After the deer convinced me to get a new '14 SGS, my two custom seats still fit perfectly!
I have 0 seat time on a new model but from setting on them at the dealer their feels to be substantially more amount of room on the 14+ from my 09. Seat may still be the same but the tour pac is moved back? Semi off topic I know