I read this thread with great interest. I own a VX Commodore and have considered coming up with a way of blocking my upper grill. I think the lower bumper opening on the VT and VX would easily be big enough for effective cooling except if the car is being driven hard (street racing) or towing heavy loads in summer.
I blocked the upper grill of my Corolla and even when driving in the middle of summer with the car fully loaded the water temp didn't increase any noticable amount.
I have contributed to a thread on Aero Drag for Holden Commodores on the "Street Commodores" online forum you might find interesting
http://forum.streetcommodores.com.au....php?t=8554029