Those 20's aren't helping either.

Upgrade to some light weight 18s, and the car will feel like a new car. Unsprung weight is horrible for vehicle performance.

 

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.

 

who'da thunk the TB spacer was screwing you out of horsepower..........
i almost bought one.

 

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.