Hi, i was surprised at your quoted figure of about 150 miles range on your 800GS, these bikes are incredibally frugal generally and i am getting nearly 200 when ridden pretty hard on mine... when i take it easier i will get about 225/235. Also, if you fill the tank from empty you will find that it actually takes just over 17 litres. Even another five litres should give you a range of 250miles pretty comfortably and probably closer to 275+.
Have you run your tank all but dry, filled it completely and then run it again till empty to check what your true range is? if not, could be worth doing.
Some day so soon....