There is a rough guess that there are appx. 30-35k GTP Comp G's out there. But some are piled up, up in smoke or up a creek. So who knows?! But you did pick the better year.

NOTE: The correct mind-set you need is how fast do you want to go and how much are you willing to pay. Figure both of those out before purchasing anything and get yourself in a hole with a pile of debt or a pile of a car.

1) Build your own air intake. ABS, PVC, SS tube, whatever. If you are trying to go for overall performance look around some more on here or on other GP sites for some ideas on what to do. I bought an SLP CAI, but the only reason was because I was going for something discrete. Just make sure you put it in the FW and block off the hole.

2a) If you really want to run one. A 3.6" is what I would start with. It's a .2" drop over stock (3.8"). General rule I have seen with the Gen V charger is about a pound of boost per 1/10" in. drop in pulley size. Given this changes with air temp, humidity and sea level o/c. I would go after freeing up some exhaust restrictions first.(See below)

2b) Agreed 100% if you want to tune. Get a HP Tuner or something of the sort. There is nothing more benificial than being able to tune your car immed after installing another mod. Consider this your most important purchase for continued modding.

2c)Plug wise, I would only drop what you need to. One or two heat ranges depending on what type of t*start you are using etc. If you drop to far you will have a problem where the plug will not self-clean because it is not warm enough and will end up fouling itself out. You can always drop more as you go with more mods. And besides, it's $10 to for a new pair. So it is not going to kill to run what is proper for your current set-up.

3) Stick with another set-up of Comp G struts. This is best set-up for these cars. I have yet to see a GP that handled better with a set-up w/ KYB struts.
Ride height to me is a matter of preference and how much of the pick-up truck look you want to lose. There is no critical height or spring rate that will let you 10+ mph faster in the corner than the next one. Lower it 1-1.5 in, get some STB and be done with it.

4) Definitely comes down to cash. There are plenty of options out there. You could go ported factory manifolds. A Plog from ZZP.(Get the front only). Or go with the headers. If you do anything at all, change the front manifold out and replace the down pipe. Those two things together are as close as you will get to hears without shelling $800 you will see noticable gains. Exhaust sound on is all subject to the ears of the beholder. For the lil bit extra, run a 3" back instead of stock 2.5" Like previously rec. check out some clips on youtube, etc. If you go custom, most people will warn you to keep a resonator in for fear of the dreaded "drone".

5) If you don't want to put the labor into a cam swap. You could go rocker arms and springs for a similar effect (don't read as same effect.) Cam would be one of the last things I would get after I was done modding up everything else and get one made for exactly what you're running.

*6) Depending on your extreme that you are going for, I would include looking into building up your tranny as well as it will be one of the first things to go if and when something goes.

Ha, that was my 0.2 cents on it... ok may be a $1's worth there...

I switched the basics on mine and am content with it. My order for a new Camaro just went in yesterday sooo... that's where I am at.