I've done some searching & still have few questions.
1. Since the Z tube helps the G breath better, do you get better gas mileage?
2. On a sedan, does it really make a big difference other than the slight sound difference? (I heard some clips on youtube & it sounds ok, I kind of like the stock sound better)
3. If I have an 06 sedan should I get an 06 Z Tube?

Get it for the sound and simplicity. If you gain anything (or you think you did) consider it a bonus.

1, no, the ecu adjusts the fuel to run at the preset AFR
2. no
3. they are cheap used, so why not

I didnt really notice any difference when I did mine but it is much smoother and less restrictive than the stock box

What is smoother? The acceleration?

eh i wouldnt really say so. the throttle response may be a bit better but it is small enough that it could easily just be a placebo effect.

i wouldnt do it if you were solely looking for gains and sound, it wouldnt be worth your money. but if you are doing it for the idea it is less restrictive and/or in my case am planning on putting a jwt popcharger on it for that sound i would say go ahead, its relatively inexpensive

OP, I have a used Z-tube... $40 if you want if you want to meet in the Long Beach or OC area

Its recently painted - was supposed to be black chrome but ended up more like black pearl - the only reason I'm selling is it because I bought an entire new one to have painted the right color (and a bit differet area)..

PM me if interested.

(for anyone reading this, sorry for OT and no, I wont ship it anywhere – its not worth enough to make that worthwhile).

That's a great deal Mike! OP should jump on this deal.