Wot, 150kg?!?! That's no WR450F offroader. I guess you mean 250kg?
Well, i've heard 1100 usually is few kilos lighter than 1150, especially the models with plastic tank. But all depends mainly on the configuration - KAT, ABS etc. Dudes say, it's possible to rip 1100 off to 208kg dry (no ABS, KAT and electrical extras).
But if you have all of them extras with full tank, plus luggage, they should be around 260-280 kilograms.

And it's not funny with it in the mud. See this:
http://www.ukgser.com/forums/showthread.php?s=&threadid=44583
Though, boxer GS has one huge advantage against weight - the low centre of gravity of all engine concepts. Once you get it movin', it handles like circa 200kg bike. And BMW engineers really think on low-centre terms on the planning table - the most massive parts are the lowest too - final drive assembly & KAT. Look at the new K1200S frame and components placing - they really are put low to the ground compared with rivals (Hayabusa, ZX1200R), altough K12 is more massive and less power, it beats them all in twisties tests show. I think the same goes to GS in massive (950+cc) travel enduros. Peak power and approximate weight really are academical compared with centre of weight and suspension technology on the solid road (duolever & telelever vs ordinary suspension).
Margus