I’ve got to agree with Locomotion. I found exactly the same results. The best result I found at the time was the taller the filter I put on the better it was. Clearance above the carb made a positive difference. In my case, it was a 600 AFB. Tallest filter I used was a 4 inch IIRC.
Engine masters did a similar test, no stub stack though, but found it the filter was taller it performed better. Check out there air cleaner test.
The only thing I’d like to re-try myself is a K&N filter lid and start testing filters at 3 inches in height and go taller from there. Perhaps mimic the engine masters findings? IDK, but I wanna try it. I thought about this before I seen them do it and had a thought that it may work. I reasoned this with the use of velocity stacks seen from the old days of racing.