looked into this when i had my non turb 20v 1.8 mk4 gti... got it lookin the nads with coil overs, audi wheels and alsorts.. but power was a no go.. espesh afgter me cambelt tensioner failed and stuffed the engine...
honestly dude, if the car aint on finance, or if it is take a bank loan out and pay it off and take extra to put with the car and px it for a turbo.. youll always be wondering why you didnt...
i got rid of mine as much as i loved it, but they have got inferior brakes, engines and gearboxes compared with the turbo model and you have to spend plenty to even try to tune em up, chips, exhausts and filters wont do alot...
if you had a bit of knowledge and peeps that knew there stuff you could get the engine to run fitting a mk2 16v dizzy, some twin 40/45 carbs, tubular manifold, and some decent cams you could probs get 170bhp ish, but to cost would out do the pros....
miss mine to bits for the way it drove, the colour, and wheels, but after 8 months of not havin a dub a jap stuff insted, my ebay £510 bargin mk3 vr6 drives better than my mk4 did.. (other than handling.. you dont buy coilovers and uprated antiroll bars for nothing...) but power wise....

lol