Tank range can vary between 160 miles,(normal riding) to 120 miles.(lunatic thrasher mode)Mine was an early 1000DS.
I never used a tank bag on mine,.....both bags I own are magnetic,....no good on the plastic tank.I have read good reports on the Famsa kit.
A briliant bike,.....I wish I still had mine,.....not sure that its any better than a ST2 though.
Loads of good info on
www.multistrada.net