I admire your needs ..
But if your worried about mileage at speeds which i rekon i know you want to travel at
you lookin at around 200 i rekon, maybe less, the sp1 gets around 160 at full throttle.
I know they are'nt in the same league but my 990 adv s with map get's about 180 k's before the fuel light goes on when i'm heavy on it.
Why not just bomb a 950/990 put safari tanks on it a gpr single system for even more room for fuel or a turbo
Or fit a duke r motor in it
Best wishes on this one