One or two naysayers in here....
Should I tell you about riding around the outside of a GSXR1000 on a trackday on my R1200GS?
Like the whitham_wannabe above said, an F800GS on 17" wheels and sticky rubber would just be a giant supermoto and we know how much they piss off sports bike riders on track days. The 800 would also lean forever. I think it'd be a barrel of laughs.
AC Schnitzer displayed a SM'd F800GS at European Motorcycle Shows when the bike was first released.