My mate had one fitted to his 300tdi and it was better to drive.
If you fit a bigger intercooler and start tweaking the pump then it should go better, it should not put more pressure on the engine if you dont drive it hard.
If the engine has more power then you dont need to push it as hard so you should get better MPG.
Thing is most people use the extra power and push the engine harder so the MPG will go down.
Because you are only going to get 20-30Bhp more from the engine, it should be fine if its serviced and and looked after.