The intake is fine.

The restriction is in your downpipe and in the tune.

Do those, that'll help. The stock intake on the GXP isn't restrictive at all...it basically just needs a high flow air filter. Everything else is just looks and noise. The intake manifold itself is the restriction. It bottlenecks down around 45mm~ after the 75~mm throttle body. Upgrading to an LS1/2/6 intake manifold and TB is where the real gains are going to be had.