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I still have mine stock, but if you do a search, you'll find a wealth of information. The ones that come to mind that are up there are Vararam, Vortex, and Callaway Honker.
I've tried a K&N filter in the stock air box, a Halltech Stinger and the Callaway Honker. The Callaway Honker is a definite keeper. Not knocking the others, but I like the engineering that went into the Callaway. It also produces the best quarter mile times and speed in my 04 Z. Another good thing is that there are no codes or check engine light with this intake, probably due to the relocation of the MAF close to the throttle body. 04 Zs are known to be finicky with some after market intakes, requiring computer tuning or a screened MAF to eliminate the check engine light and codes. The bottom line is that any of the cold air systems you mentioned will produce more horsepower than the stock intake. My vote goes to the Callaway Honker.
I Installed a VaraRam Vr-2B on my '04. no codes thrown. Took engine less than 5 minutes to learn it. Very noticable difference on highway runs. I even considered removing my screens to ram even more air, I decided against. Stick you hand out a window at 60 mph, palm into the wind. Now think about how much pressure and volume forced into the throttle body. Power is instant. You must do before and after runs to appreciate it. A ZO6 is strong stock. Anything you do can only help.
Everyone is pretty much going to recommend the intake they have. Any of them work well and all are an improvement over the stock setup.
With that said, I have the Vararam2b and am completely satisfied. I can totally feel the difference at highway speeds and just the ram air set up is pure genious and with the front grill vents, it's almost as if the car was meant to come with the Vararam
Running the BPP vortex rammer here, love the look and great results. Of course, they all do well and will all make good power. The new Callaway does show impressive engineering, and the Halltech Stinger is super easy to put on.
My only gripe w/ the Vararam is that it blocks the cold air screens which is bad for road racing/HPDE use... those allow air into the engine compartment to sweep away hot air, the Vararam blocks that.
Hurricane intake here. Got it in C/F - looks awesome! Definite seat-of-pants gains. Mine mixed in w/LG longtubes w/matrix cats and x-pipe and B&B PRTs. Fabulous sound at WOT!