The folks I know who ride Killers, get mileage from upper 30's to upper 60's depending on speed and conditions. They dont lend theirselves to hopping up, you generally do nothing but kill your mileage and make it louder. They have a super support network. You can find everything from custom long range seats to 7 gallon fuel tanks, taller windshields and custom altenator windings to give you a little more juice.
No matter what, make sure you do the doohickey modifaction. (cam chain tensioner, thats what its called LOL) Doohickey dies, best case your walking, worst case your motor will shell out.
http://www.klrog.com/TheKLR6502.asp
http://www.klr650.marknet.us/