Hi mate,
Sorry....are you saying that you have a MY03 WRX and wondering if you can get a bigger turbo?
If so, the answer is a most positive yes and it will make a difference.
I started with a TD04, moved to a VF34 and now have 334bhp/335lbft in my WRX.
I believe you'll have a TD04 and you can really get what you want.
It all depends on how much bhp you're after and how much you have to spend as you'll need
to buy supporting mods and get a remap etc.
The guide here will give you an idea
http://www.andyforrestperformance.co.uk/58901.html