would it be good for airflow to put an additional fan in the top rear of the case?
how much exhaust you may need really depends on how much cool air intake you have,
how much heat is being produced,
and how efficient your current cooling setup is.
example:
if you've got two high airflow\air pressure intakes running at 50%
and two of the same fans running at the same speed for exhaust;
you would have zero interior air pressure and the cool air would be sucked directly out of the case with no chance to flow over and cool most components.
if you set the intakes to 100%
while leaving the exhaust fans set to 50%;
than you would have a positive air pressure and the cool air being pulled in would have more of a chance to fill the entire case, cool VRMs and other components, and push any warmed air towards exhaust points.
of course it's different in every scenario with different coolers, different fans, and different case designs.
all of the system build info would be necessary to determine if it would make much of a difference for your particular setup.