As far as fuel mileage is concerned, my brother reported much better fuel consumption on his R1150R after we fitted the sprockets, head porting, removed the cat and chipped it.
He found that one gear higher everywhere incuding top gear rollons for overtaking even two up helped out.
We also took a stock R1150RT and the guy ran the big bore headers, Staintune exhaust, sprockets, InDuct (large bore carbon intake), FIM chip (for the R1100R) and a K&N filter.
I rode the bike as the owner lived near my brother in Queensland whilst on a visit and after owning a much modified R1100S I was amazed at the mumbo this thing had. I have ridden the RT before and even two up with my son, the acceleration in the first 4 gears and then top gear roll ons was amazing.
He also reported one gear higher riding and much better fuel consumption.
As a side note, to date I have not had one pair returned.