Highly recommend Rays MAF Amp, we have it fitted to our TDI, and I love it. You can really feel the difference between when it is turned off, and on, and the car pulls so much better up the hills with it.
On a very unscientific experiment towing the caravan down to Wales at Christmas, I had to change down far less often than I have on the trip before, and one hill, where I used to have to drop down to 3rd, I did easily in 4th, and for most of the journey could stay in 5th.
I think it does result in us using a little bit more fuel when it is on, we drop from about 420 miles a tank, to 390, but even that is hard to prove, as we also run Veg oil from time to time, and do a lot of vary varied journeys, from a week of short runs, to a long drive, and then back to short runs, so that also has a huge effect on the MPG, but the extra power is so worth any loss.