You don't need full scale wind tunnel tests anymore. Some wind tunnels ( very expensive ones; perhaps we can start a thread on China's wind tunnels and other test infrastructure ) can vary the temperature as well as the air speed so that for a scale model both Mach Number and Reynolds Number can be equal to that of the full scale aircraft in flight. But for cruising flight of transport type aircraft and perhaps by now for more difficult cases computer simulation of the flow field around the aircraft can be very good indeed.
Someone do please start a China's Wind Tunnels thread!! I've read so much in piece and bits about China's gigantic wind tunnel groups for the last couple of years. I think I do need some cool-headed discussion on this subject and determine where China stands in such developmen.