had exactly the same issue with my girlfriend. the GSA pillion seat was awful for her, the problem is they are very flat. the GS seat is more shaped and slightly thicker which gives a tad more legroom. the front and pillion from both bikes are interchangeable although the front on the GS is lower than the GSA by about an inch I think, not completely sure about that. I bought second hand off this site, quite a few come up for sale