I know you will have more power from a 1.8t but they ain't as reliable. Well they are I'm just saying that you got turbo and turbo parts to fail now.
Also will cost more to fit a 1.8t in a 8v mk3
Ain't nothing wrong with the power of a vr6 but they are bad on fuel.
Me personally would go for a 2.8 24v found in the mk4 4motion. You can pick one up for £1000. Take what you need sell the rest to fund for a vr6 box, lightened fly wheel and a remap. Should make 230bhp.
1.8t is only worth doing if you get a BAM or AMK k04 engine and it cost more than £1000.