75 -80 is a cake walk on a GS, I had mine up to a buck ten last weekend two up. 80 mph all day long on the slab is absolutely no big deal. Just like the big pig, you will probably need to deal with the seat and windshield. I had an F650GS and sold it after 5K, not enough power and more buzzy than the 800GS.
I put a laminar lip on mine which makes it quiet to the point I can listen to my tunes barefoot from my zumo with no amp or intercom at 80 next to a semi. As for the seat, I'm happy with the stock seat.