It will make your engine MUCH slower. The Garret diesel turbo has infinitely variable vanes that adjust boost output according to engine demand. But, if it goes too high or too low the electronics will cut the fuel until the pressure drops to ok levels. This will kill perfomance and mess up the mpg.
If you want power, get a chip upgrade. See the Superchips website. Not cheap but they cover for any warranty engine failures caused by the chip and they have been about for donkeys years. Make sure you chat them up about who to choose for fitting the chip.