How much HP can I get from this STi setup?

I would think you can run a little more boost like a pound or two but I wouldn't raise it much more than that with the stock exhaust. Especially the down pipe with the stock cat in place. Hard to say what you are making or what you can make with proper tuning since you are missing some parts to start upping power but I would say you can run somewhere in the 220 to 250hp range with 10 to 12 psi or so.