I installed such a fan after an under panel exploration with a thermocouple showed temperatures of 140F while the cabin was comfortable.  I used a fan about 3.5 inches square installed above the radio stack.  It blows on the windshield, but more than a few inches from the exhaust, the flow is undetectable.  After about a half hour or so the air coming out varies from warm to hot when you put your hand directly over the fan.   It shows that the combination  of electronics and (probably more important) heat leaking through the firewall produces a lot of heat under or behind  the panel. 

 

I don't know how much the fan helps, but it must help some.  Nonetheless, after an hour or more, and with an avionics fan blowing into those avionics equipped with an inlet port (e.g. Garmin 430) the bezel still grows warm/hot to the touch.

 

Modern avionics can operate at elevated component temperatures, but reducing temperature can only increase lifetime and reliability

 

Fred