After reading several threads about improving performance on the GXP it sounds like the Zex nitrous kit is the only way to go. Anybody have any other new ideas?

No new ideas. We have beaten this horse around for 3 years here. The PCM is locked tight and in that 3 years no one has come forward with a doable one other than nitrous. 2 thoughts I had were an STS rear mount turbo, expensive and no one knows how it will do not being able to reflash the PCM. They dont make one for a NorthStar so the univeral one would have to be used and cobbled.
The other is putting in custom Northstar PCM used in Custom high end street rods. Very expensive ($1800 for a Magneti Marelli), wiring issues, then it has to be custom programmed..... I choose wet Nitrous. The engine responds well to a 75 shot, not so well to a 100 shot. The only alterations were a switch to copper finewire plugs. Nitrous eats platinum for breakfast.

a rmt turbo would be easy enough and then use an afc and a Split-Second Fuel Injection system and your done. engine should be fine up to around 8lbs by my guestimate. probably do it all for around 2k if you install it yourself.

I'd suggest gutting the airbox. I don't have any baseline testing to prove this will help performance. However my car has the quickest track times of any GXP I've seen, so I think it'* safe to say that at a bare minimum, it can't hurt. After carefully examining the GXP airbox, I feel compromised for quiet operation as are most modern airboxes. No matter how you slice it, a 2 and 5/16" tube is not the optimal size to feed a 4.6 4v motor.

My sig contains a link with details on how I initially gutted mine, and that is what I had in place when I ran 14.60s. Since then, I've made a few more changes to improve flow:

1) There is an unused bracket somewhat blocking the main inlet that goes to the fender. I removed it.

2) Part of the dividing wall was remaining on the upper half of the airbox. I opened that up a bit more.

3) There are some sharp edges near the outlet where the air filter lid meets the MAF. I cleaned those up with a grinding bit; 5 minutes, why not?

4) The main inlet to the airbox is 2 and 7/8". I opened up the hole in the box and replaced it with an angled PCV adapter that is 3 inches. I know, marginal improvement, but since I am going to be building a FWI, the main inlet was going to get opened up regardless.

I am planning on building a proper 3.5" FWI. The biggest challenge is getting a large enough cone properly located in the fender.

I agree that a rear-turbo is a possibility. However it is not going to be quite that simple. At a certain level of boost, you are going to peg the MAF meter, and the PCM is going to reject the signal as false data, and put the engine into limp mode. A MAF Translator with wide-band o2 input could possibly solve that problem, but you are talking about a lot of hours of brainstorming and tuning to get it right. Also, I think you'd need to avoid any significant boost while the car is in closed loop, or again the PCM might recognize this as a fault.

hmmm...good point.i forgotz about teh limp mode. no way to bypass it?

has anyone gotten a real engine management setup to work on your cars? like greddy emanage etc.