Snip
That's exactly what I HAD thought, until I was told that the air could pass through too fast and not pick up as much heat.  Mark... this is what I was trying to communicate. It could be totally wrong so let's get more opinions.  Ed??
 
*With respect to exit area size only.  *
 
If the volume of air through the coolant radiator was moving faster than *optimum* the air delta T would be lower than it would be with slower air. Slower air at a higher delta T means less air needs to exit the cowl.  What is optimum? Every install is unique so it needs to be viewed \ identified on each installation. Ed uses a 30% duct air speed as a reference point. If I understand this correctly the diffuser play a role in duct airspeed. If the air is not being diffused optimally the airspeed could be much higher than 30% through the part of the core. Dennis H. and I observed airspeeds through a test core at 50% + without a diffuser.
 
Your inlets are 72sqin with 306sqin core face for water and 24.75 sqin with 102 sqin for oil. Both are *competing* for the same exit area. *IF* this is an exit area problem only then enlarging it should improve both water and oil proportionally. Water should realize 2/3 of the improvement and oil 1/3.  306 sqin vs 102 sqin. Your improvements after opening the exit seem to track this very close. So increasing your exit area further should show more improvements in both oil and water to a point. But you do not need more water cooling improvements right?
 
Core 306 sqin at an airspeed of 115mph.
 
40% or 46 mph = 8602 cfm   8602 cfm at a 50 deg air delta T = 7741 btu's \ min
30% or 34.5 = 6451 cfm        6451 cfm at a 80 deg air delta T = 7741 btu's \ min
 
In this example 2151 cfm less air needs to flow through the water radiator to produce the same btu rejection. So what effect would 2151 cfm less air through the water radiator and exit area have on the oil cooler's ability to flow more air? Would it improve the oil cooler air flow by 1/3 or 717 cfm? Not sure.   Can your radiator produce a 80 deg air delta T? It may only produce 60, 70 or??
 
Am I in left field here?