The Airheads are not particularly fuel efficent, lots of volume in the dated combustion chamber design.
My low compression '92 R100GS averages 37 to 39 M.P.G. in my mixed driving commute, but can do as well as 44 M.P.G. out on the road.
I don't believe the 800 head will work on the 1000.
Seems a shame to take off the RT fairing, it was one of the best ever made.
The best place to learn about Airheads is the "Old School" section at advriders.com
Playas del Coco
"..an attaching bike on which one rolls serenly along."
(Google translation of a French R100GS review)