keeping in mind that I know very little about cars, and NOTHING about Subaru's... i'd say get a good tune and that will change a whole lot about the car. Without looking at all of the numbers, the GXP has 260 hp and weighs 2948 lbs. The WRX has 265 and a weight of around 3200 lbs.. 250 lbs can make a difference.
I do know from owning a 08 Evo that they made the thing soooooooooo rich that it really slows the car down. Of course that is sort of their safety net for people not having blown motors (although Subaru blew enough motors in their early 09 WRX's lol... sorry quick poke). Get a good tune and see if that doesn't change things quite a bit.