Don't bother spending money on things like transmission.....are you actually considering changing the whole thing or something

....you shouldn't need a heavy duty clutch in a 2.5NA either. When it comes time to sell the lib, you don't want to be trying to sell a modded 2.5NA.
Front and rear sways, bilsteins and phat GT lower control arm bushes. approx $1000 all up including install.
You will probably regret spending any more then this in the end, especially if you are going to upgrade when you get off your P's. At least with the sways and the lower control arms (I'm assuming you could even switch the arms between the cars....but it's just a guess) you can just swap them over to the GT, also the billies if you don't get a Spec.B....but you would be silly to not go for the Spec.B because you would be missing out on the STi 6 speed manual.
Basically, just enjoy your car for the next 1.5 years and make the most of it and don't waste your money.
It's worth the wait, and then you will have more money to spend on your GT, because you won't be able to recoup the costs on the mods that you spend on your current car.