It seems with Z's, the more heavily modified they are, the worse they do. The benefits from whatever mods you make don't keep up with the harder competition you run against. So if you can get back to stock class running on stock wheels (wink/wink TT rears all around on R compound) you probably have your best bet for being competative. For what it's worth, I've been able to consistently win the --granted-- novice B stock class over a dozen or so others in a very competative SCCA region.
Where I'm going to find another 2-3 seconds to be competative against the slew of S2000s in the real B stock next year I don't know. I'm hoping that going to R compund tires will at least allow me to finish somewhere in the middle. The way the car is currently classed, I don't think there is any way a Z will ever be competative in AutoX, at least in large regions like Northern Ca where there are lots of good drivers with newer cars like S2000s and Boxters.