I wouldnt use one of the cheap "chips" off the internet.
I use the term "chip" losely as it is a resistor in a box that tricks your car into thinking its running really cold so it ups the fueling all the time NOT GOOD

If you get a proper re-map on the other hand then it adjusts the parameters on the cars ECU so it will still rev smoothly and cleanly. if you drive like you did before, you should get as good if not better MPG.

It is the torque on turbo diesels, my little ibiza TDI is now running about 200bhp and 300lbs and it is an animal on boost to be fair.