4-6 PSI can get by without an intercooler, but
once the Turbo Bug bites, you will want to bump
the boost, so install and intercooler now and
save the rework.
You might need bigger injectors, but the stock
pump should work fine, the computer seems
to cope fine on a 16V MAF system, the MAP
system is another story, I'v added extra injectors
and a secondary EFI computer to handle the boost
chores, I would recomend 30-35 PPH injectors.
As soon as I can afford to, I'm putting on a 3"
exhaust system, 3" Cat too, about 300 bones
installed, 2" will work, 3" is better for turbo only
not for the N/A motor tho, so keep the old system
should you ever go back, or need to.
I think the WRX Turbo is a little large, but will
work, what size is the WRX engine ?  and what
boost do they run, these new Turbos are all
internally wastegated and will take some re-working
of the wastegate actuator to reduce the boost, like
a new can for less boost.
Stainless Exhaust Valves are a good idea, and lower
CR pistons would be a nice change, about 8:1 is good,
stock Cams work very well with most Turbo applications,
if you get too radical of a Cam it actually is worse for
power, not better, remember you are force feeding now
and don't need the radical overlap, to ram air/fuel into
the cylinders, and as always, a little head work will improve
breathing, but too much will slow the intake charge and
reduce power in non-boost conditions (off idle to 2000 RPM)
so port a little if you want, but don't go hogout crazy
Let me know how you do with it
Wild
