Wow! another N* owner to talk to.
Andy theres a couple things you CAN do.(not much but..) Start with swapping out your OEM Air filter (probably dirty by now) with a K&N filter # 33-2063, might have to ask for it by part number/ most parts catologs dont list it for GXP.
When you add fiter you'll notice a tube in the first chamber/ gut it. Have someone rebore your Throttle body to 80mm. Theres a machine shop listed in this section and others.. I've researched extensively and not much else can be done unlees your want to rip out your near new exhaust system.
I am nitrousing mine,....but ran into a brick wall with the Holley/NOS setup after they said it would work.....and didnt.(But they did let me return it after half of it was already on car. Now have a ZEX nitrous kit on the way and it looks like a shoe-in, ZEX #82023 (even has an adapter to screw into fuel rail)
I was going to gut my airbox but the Nitrous / gas jet must be 6" in front of TB so I'm removing the airbox entirely and going with a 9" chrome tube between the TB and a cone filter which will set at far right behind drivers fender well. I will post pics when it'* all done. T he kit I mentioned can give you 75-150 Hp boost for $600 and some change and you can go postal with a rejet kit for 150Hp-250Hp.....but I think the engine would fold up at that kinda boost.
The great thing with nitrous is it is passes undetected though the system and wont need a PCM reflash which cant be done on our engine. Also our engine has a detonation(knock) sensor in head between cylinders 1 and 3 which will retard timing if the nitrous mix starts to detonate in intake( A super big plus for a nitrous setup). Read through the FAQ'* listed at NOS and ZEX before you listen to old '70'* horror stories from people who have never used. The systems these days are a far cry from the past and the only real hope you have to make that GXP fly.