Tire preesure for GT3
#2
For street, you can add a bit to the fronts, maybe 34. Other than that, they work well for the street at those pressures. Car will have just the right amount of understeer to keep you out of trouble.
#3
IMO, the stock tires work best around 40psi hot front and rear. Since they will warm up about 4psi during normal street driving, try 36psi cold front and rear. For aggressive street driving, they will warm up more, so start out a little lower. Check hot pressures after your drive and adjust accordingly.