This is a complicated question because it's not just a matter of adding a chip. First of all, you need to tak to people who have done a chip already and find out what problems they incurred. For instance, if the chip puts more fuel into the engine you're going to need an exhaust gas temperature gauge for SURE; and you're going to be blowing out lots of black smoke (you can pour as much fuel as you wish into a diesel engine---if it doesn't burn it, it just blows it out unburned), so you'll need to modify your intake as well.
What I'm saying is this is not "plug and play". You'll need a plan, a budget and some first-hand information from users.
Also, as for mpg figures, don't go by what the car's computer tells you, since the chips change those readings.