TBH, you might as well start out with a VR6, or a 1.8t.
as your never gonna gain a big enough power gain that you probably want, you will always be chasing the "dream".
IMO and experiance, you have to start with a good base to tune.
the 1.8t engines are tunable to hell. and have forged cranks and internals as standard. so can take massive BHP with out questions.
as said earlier on the in the thread, your only gonna gain small BHP for alot of work.
Sure you can add a 1.8T...which in 150BHP form is slower point to point than a 16v with 150 BHP.
So you turn up the boost...giving big torque...which the ancient Mk3 chassis can't put down so you end up with a wheelspin machine, whilst the 16v with less - and more linear - torque delivery drives away whilst being easier to drive.
If you fit a K03 1.8T the weight is similar...but if you fit a K04 1.8T you have more weight, and in order to not eat gearboxes, you probably fit the 02M gearbox (or build a hybrid 02A with Quaife/Gemini internals at a cost of £1000s)...so you add even more weight to the nose (02M) and end up with the poorer handling associated with it.
200 BHP from a 16v is not cheap or easy compared to as 1.8T...but it's much nicer to drive, in my book.
Horses for courses...
