Since pressure is proportional to velocity squared the pressure corresponding to 75% IAS is .75 x .75 = .562 of maximum dynamic (pitot) pressure.
The lower cowl pressure corresponds to .122 of max pressure, a difference of .44 of max dynamic pressure.
This assumes that the ASI static port was connected to a good static system. If it was open to cabin pressure, which is usually below static, that would make the upper pressure look better (higher) then it is, and the lower pressure worse, (higher) than it is.
44% of dynamic pressure at 200 MPH is equal to 100% of dynamic pressure at 133MPH, and since airplanes cool successfully at slower speeds it should be enough, are you sure the temp probes are calibrated?
To get more
pressure there is more to be gained on top than bottom. Larger openings or diffusers aft of the openings would help, but I would start by eliminating leakage and making sure the baffles force all the air through the cooling fins, and that no gaps can open up under pressure, in flight.