loss of torgue.
 

what would cause a substantial decrease in the amount of torque my car produces, she still hits full boost just 1500-2000rpm later and builds up speed naticably swoler than usual.

It would probably be an air leak post turbo. Causes an overly rich mixture which reduced power and torque, although full boost can still be achieved by over spinning the turbo.

Any louder than usual hissing noises coming from under bonnet?

nohissing noises that i can hear but i'll do a pressure test on my ic lines to be sure.
exhause is not louder also.
post turbo meaning header leak or all ? but then she wouldnt be making full boost if it was a header leak right..

cheers.

AVCS Solenoids maybe ?

What year and what oil are you running ?

03STi running what ever ozy park put in it.

torgue?

will i need to block off the throttle body to compress ic lines?

Try seeing if your BOV has seized up if it is aftermarket...

I had similar torque loss / boost loss and then Sean pointed out that the BOV had paint on it and had probably seized up which it had.

Once it could function again car is perfect. It was just stuck open.