I drive my z in hot oklahoma weather 105+ both my old na and the new TT and they perform just find with stock cooling systems. granted the tt doesn't pull as hard in the heat but it's not like the car will melt over the 95 degree mark.
As for the rain... well.. rain and me being a stupid kid brought a early death to my NA -_-' so i would avoid standing water at all costs. However if it does rain the z is still a car so rain sleet or snow depending on the tires that are on it determines on if i use her.