Ok so I installed the valve (NAPA 6601413 $21) and did some driving with temperature readings. These numbers are with minor rounding as I was dealing with driving in some sun and some shade as well as a slightly rising morning temperature. Doing any testing in park with the hood open will produce bad numbers as the air intake along the windshield just sucks in hot engine air. No picture as this is so simple a picture wouldn't provide much more information.

Ambient Temp - 74F(shade), Upper Vent Flow, Fan 3/4 Speed
A/C Off/Outlet Air:
Valve Closed - 82F
Valve Open - 82F

A/C On/Outlet Air:
Valve Closed - 46F
Valve Open - 46F

As you can see there is virtually no difference with or without the water valve. For sure it cuts off hot water to the core, with the valve closed I got no heat when temp dial was set to high. But the heater hoses at the firewall were the same hot temperature, maybe because there was still flow to the throttle body and that was enough to heat the hoses.

The center console pocket was cool with the valve closed so there is definitely less heat in the cabin to cool, but over all Mitsubishi did a great job just using the blend doors to distribute heat/cool.

You can draw your own conclusions but this little project did not produce what I anticipated - about a 5* difference in A/C air. There is less heat in the cabin, so that might make me feel cooler, but the numbers did not show this to be a worthwhile modification. I'm glad I only chose the $21 valve and did not go with a more expensive remote control one. You also have to understand that the compressor cut off temperature will not change as this is determined by the temperature sensors in the evaporator box, maybe this is why the outlet temp is the same.

Hope this has been helpful.