IMHO, if you are going to run ST, STX would be better than STU even with the tire limitations. At a purely raw level, the STU Mustang should be faster than the STX Mustang, but the competing cars are much faster in STU. 285's and the other legal changes are not going to be THAT much better in STU versus STX that you could run with STI's, M3's, and Evos in STU.
The PAX difference is about .02, which is huge in autocross. The PAX is just the PAX, but it does do a decent job of reflecting performance differences between classes.
60 second run in STX pax = 49.32
59 second run in STU pax = 49.619
You would have to run 58.64 in STU to match the STX PAX for 60 seconds.
Since cost is also a factor, I would stick with STX even if you could run in STU.
The other thing I would serious consider if I were you would be to purchase some cheap 18x10 rims, some used wide A6's and give ESP a try. You can get into used A6's for 125 bucks a piece (or even less). If you could find V710s, even better at least in terms of durability. You will not be as fast as you ultimately could be with fresh tires, but it will be close. Race tires will completely transform the experience. I miss running on race tires. The only reason I run road tire now is because I have been bring my son to events with me and, frankly, I am too lazy to switch out wheels at the end of a long day.
ESP is really the modified V8 mustang's autocross home (unless you ramp it up to CP).
Just my