Hi Tom,
I would look at either a reflash of your stock
ecu (ecutek (MMS or Racetorque only in WA)) or a direct replacement
ecu (Link, autronic, motec, power fc Etc (depends on your tuner, needs and budget)).
One of the main reasons for this is that the sti
ecu's were tuned for the higher octane japanese fuel and a few of the imported cars (the limited run of 2dr and 4dr sti's that came here in 99/00) experienced piston problems due to this tune.
In terms of selling your car and buying an sti..... why? You have such a nice example there, keep that and develop it further. Others may argue over better this and that, but when you have a good car, you can make it great. You will find that most sti's in the 99/00 year group will be nicely thrashed at this age (you don't buy one to baby it around) and in terms of potential, the wrx and sti's were near on identical.
For your target power level of 140-150kw atw, the td04 will handle this sort of power and be responsive down low, just dies in the higher revs (above 5500) where as the VF turbo's will keep going but lose a little of the down low urge.
My car runs the stock td04, 3" system and link
ecu and has made 150kw at the wheels, it is smooth enough for cruising, has done a few track days with 2 drivers and very little rest and been used for drags and mc work too. Never missed a beat thus far. You are welcome to have a drive if you like, if this will help with your choice.
Hope this helps.
Toni