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I do know the early cars (97-98) had bigger injectors, as Lingenfelter charged extra for his first twin turbo kits when installed on later cars as he had to "upgrade" them to the earlier injectors.
BTW, I've always understood that's the reason early cars get more benefit from a CAI system than later ones - look at the K&N web site for their dyno numbers as an example.
p.s. Wouldn't it be great if all CAI vendors published actual dyno results? It would get rid of a lot of the mythology out there.