Re: 300 hp wrx?
Your dealer is a tool, and has no idea how the wrx boost control system functions.
A stock WRX runs between 13.5 to 14.5 psi max boost pressure, tapering off to 9-10psi-ish at redline. Depending on the dyno, stock WRX's have dyno'd at between 165 and 180 wheel HP.
You can use any number of aftermarket boost control devices to turn up the boost pressure, but the stock turbo on the WRX is fairly small, and is only good to about 280 crank HP, which should be around 230 whp.
There are several bolt-on aftermarket turbo options available for the wrx. The largest of these should yield (with proper tuning) around 500 crank hp. The stock fuel system is safe (<85% IDC) to around 320 crank hp, but people have pushed it beyond that.
Anything you want to know, just ask.
02 WRX/TEC3 Engine Management/VF34