I have a 2001 Merc e320 cdi and fitted a Van Aaken box (Van Aaken developments) about 35,000 miles and 4 years ago.
The power went from 197hp - 239hp the torque went from 347-380ft lbs
The 0-60 went from 7.8 to 7 flat , which is pretty good for a diesel imo especially a diesel estate.
Fuel consumption went up on average 1 mpg to 34.5 which isnt a lot but i dont do many long trips , it gains perhaps 2 extra mpg on longer journeys and cruises just a little better at motorways speeds up to about 42mpg.
Paid for itself easily , well reccomended ..
Not a bad word to say the car has been fine , no detremental effects as yet only downside is rear tyre wear increased a bit
Heard good things about the Tunit tuning box as well