if you are running a stock turbo (td04) then you should be able to go up a couple of psi without too much hassle. also, it may be difficult to tune the stock computer (presume it is) for greater efficincy with fuel unless you can get someone to ecutek or verso it when this becomes available.
I have everything stock with the exception of a full exhaust, intercooler spray and link
ecu. stock the car ran 12-14 psi depending on the weather. with the link installed it ran 17.5 psi with the spray going and made 150kw at the treads on a roller dyno quite late in the day and in relatively warm conditions. poor economy too.
with a tune, the boost was dropped to a solid 16psi and power was the same with much improved economy.
given that you have stock boost levels now, a couple of psi extra should be no problem.
if you do go for a larger top mount then i would suggest either an aps unit or a hyperflow/pwr unit. both are reasonably larger than the stock one.
don't be fooled into buying the same year/model sti ones because they are exactly the same item. bug eye onwards are larger but will require some fabrication of brackets and a trim of your master cylinder to fit correctly.
top mounts are also better for turbo lag because you don't have a long set of pipes from the intake to the inlet manifold.
you will find that your stock turbo will start to kick in at around 2500rpm and die around 4500-5000 rpm. I found this out at the drags as the turbo simply is not efficient at high revs. the td04 is the smallest turbo fitted to a wrx and was primary chosen because it was not laggy and provided low down power. the larger ones come in later and last higher in the revs, producing more top-end power. hard to find a balance of a turbo that comes on early and holds high up.
hope this helps.