I guess there is one thing that everyone is sort of forgetting about. If you really want to save fuel, and put out insane numbers, its going to happen at interstate speeds. This is where the Z fails, because of its poor aerodynamics.
Take a look at any high mileage production car out there, and there have been sacrifices made for aerodynamics. The CRX hf had no hatch wiper and no passengers side mirror.
The insight has EXTENSIVE aero consideration.
Personally, if I was hunting for a car to "hypermile" in I would go for a Geo metro.
as a side note, I was totally anal about logging all my fuel mileage numbers (i'm talking excel spreadsheet with correction factors here), until I realized that I wasn't having fun driving my car anymore and all I could think about was how poor the mileage was. The next thing to do, for me at least, was to stop logging the mileage, and not think about the mileage.
Now I just enjoy the ride.