I've done before and afters on dozens of bolt-on GT's and I can say from my experiences that good CAI's (not fenderwell models) that use the stock MAF are good for 1-5hp on an otherwise stock car. TB gives about 3hp, add a plenum and you're up another 5-8. All 3 together usually get 8-12 hp. They reduce restriction a little but the real restriction is as has been said many times more in the heads and cams than anywhere else. If you stuck the 600 bucks that those 3 bits cost into cams then you'd be up 25hp or more.
4V cars, particularly the Mach1's have been known to pick up 15-20hp and more with the intake upgrades.
Upgrading to a larger MAF is almost always a bad idea. Blower/turbo cars are the only ones that I'd even consider it on and those have to be doing 400-425 or better just to be candidates. The rubber inlet tube is a source of perturbation in the air stream which lowers sound and power and that's the only part that I'd think of changing, and I'd change it specifically because the mod motors love RPM and high RPM needs smooth air for max power and safety.
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