hi i have an alli sport intercooler on my 300tdi ( the biggest one they do for it ) and its made a world of difference. it pulls better, is smoother, a dont have t change down much at all when goin up hills. It was worth every penny.
I also have done the de-cat, removed the egr valve, tuned the fueling up, fitted silicone hoses, and have put on a new rad n a electric fan.