Theres two/three ways of doing it, either transer box, or diffs, or like said with the power ( fueling ) depends on how much you want to spend or how good at workin on the thing
You could get alot more poke by changing the rads to higher output ones or wind the fueling up, at some point i think someone has messed with my fuelin to get more power because with like for like tires i can out drag, my dads td5 and iv got a 300tdi, even more so on the uphill motorway runs
It dosnt make much diffrence to the fuel costs, its always going to be naff from the off, but we dont buy these things for eco runs firstly though make sure your turbos in good nick because thats something that should be kept in tip top condition, not just for power but your yummy 300 engine
Daniel