The Z is good at the auto-cross, I have not been beaten by an STI yet - even in full wet conditions. Obviously it does take more skill to drive a RWD car in the wet vs AWD but the car does perform well.
Tires are a big issue in the 19" sizes, I am buying 18" wheels to widen my tire choices -
see here
You can just barely cram 4 wheels in the car with all the stuff in the rear removed by stuffing 2 in the back and 2 in the passenger seat. It's not very easy to drive this way as you can't see out of the passenger side of the car at all. I plan on having someone with a full trailer carry my wheels for away events, for local events I will drive as it is close. Eventually someone might come up with a trailer hitch capable of towing a small tire trailer like this:
The car has some serious issues under track conditions that is going to cost a little bit of money to fix.
1. The car needs an oil cooler, you may not need it for auto-cross but you will absolutely need it for open track.
2. The brakes. They look OK on paper but fail under track conditions. We're still figuring this issue out but it would appear as though the rotors are overheating. I've never seen it this bad on a stock braking system before.