I've just fitted a tunit tuning box to my golf gt tdi 130, i thought I'd let you all know what its like because everybody talks about revo etc.
to be honest i can't really tell any difference below 1600rpm although it does feel a bit smoother and slightly more responsive, but when you get it up to around 2000 rpm it really goes well, alot better than standard. it's suppose to give around 30 extra bhp(apparently thats a conservative estimate), i honestly don't know how much more power I'm getting because i haven't had it on a rolling road. but i can recommend a tunit. also the fuel economy is slightly better.
you can fit it in 5 Min's, just disconnect battery connect a plug and socket into wiring, and also connect it to a 12V supply.
they have a website, but i bought mine from a guy on UK mkiv forum, for a £100.