When I go to the track if I run on my street tires (Nitto 555 283/35/17) I usualy leave the air pressure at 30-32psi. I would feel safe to say that you can run ZR's down to 25psi, but what I would do for the first couple of runs is have somebody watch the tires to see if they crinkle real bad or not. With low air pressure you run the risk of your tires "wrinkle walling" (think low pressure slicks on a hard launch), and if your tires should do that to the extreme you could cause the tires to unseat thereselfs on the beads and blow out. I would feel comfortable with 25psi though. I'm really not sure on the construction of the Bridgestone Re 730's, but I also have a set of Nitto 555'R D/R's 275/40/17 that I have run as low as 16psi cold (before burn out) and had no problems with.
------------------
2000 Mustang GT