I've bought my GSA in 2010 as it was an unsold '09. The seat and I were not the best of friends. I've got a bit past 24,000 miles on it now and it seemed the comfort range was about 400 miles when I was always on a 500-miler.
I had an '04 GS and that stock seat was the most comfy seat I've ever had on any motorcycle.
Rode down with a buddy on his F800GS to see our dads in SoCal. On the way back, we decided to push it. So Tonopah, NV, to my place in Bend, OR, 625 miles in 13 hours, three rainstorms, and one hail storm.
Amazingly, I was not sore one bit. So either the seat is breaking in or I am. Guess I will take the money for my aftermarket seat and use it for better lights, instead.
And I can tell you an extended run of 80-90 mph will drive your guess mileage down to 32 mpg.