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That's been mentioned, but what strikes me more, is that changing to 17's didn't feel any different. Kinda suggests the OP's butt dyno is completely off.
yeah i wouldnt bother with it. my car felt a little better after i removed it, but it still feels sluggish and less torquey than other z's. my friend has a bone stock z, and no matter which one of us was driving what car, even when i was totally stock with a plenum spacer, his pulls a little on me from a roll.