this is somthing that i was very concerned with when i got my 1st rs about a year ago for me running a std rs with a stainless exhaust and a k&n i found i was getting beween 70 and 75 miles to 10 quids juice.
after a few mods (stg 1 165bhp@14psi) i was getting about 60 to 65 miles for the same 10 quids worth of juice now i'm running a 180 bhp 1.8 zvh and can care less as once the turbo bug bite gets a hold you cant stop!!!!!!!!!!!!! i'm now about to fit a 2.1 zvh with a gas flowed head, piper cam and fuel costs are now long forgotten as its the buzz thats takes over and after all there plenty of loan companies willing to help out.
Besides i could sell a kidney on the black market and maybe look at fitting a 2.3 with gallons to the mile not mile to the gallon