I have done this, upgrade as I would put it & would not go back
My MK4 130 was quite stock so what I noticed
The Mk5 GT TDI 170 feels rock solid to the MK4, Shocks, Steering, ride is firmer I love it
Power the MK4 130 is nicer at 1500rpm pottering around, but over all power the mk5 170 is a feckin Rocket. I still have not put my foot down fully I love it.
Not a cheap upgrade though, oh check your insurance though as I did get told on the phone "thats a quick TDI" & I now pay £540 fully comp from £300 for the mk4
Jay