I would hope that if you're going skiing in the Z, you have your heater hooked up! With the heater on, hot coolant gets shunted from the back of the head to the water pump inlet thus performing a similar function to the manifold loop. Now, assuming that the heater is connected and functional may be a big IF for Z-car owners (including myself ).
I think icing is the big issue at play here, so if the car is only driven in warm climates, removing it doesn't cause much issue. There will be no water pump cavitation if the HVAC control is switched to HEAT while the car is warming up. Warm-up time will be affected, but not too badly if the temperature is already "warm".