Well there is only so much theorising you can do, sometimes you just got to go and try something and see what is going on.
Off on a tangent, I worked for a company building replicas of the Jaguar SS100, these were fitted with an XK 4.2 lump. The cars had twin SU carbs with individual mesh filters, they would not rev beyond 3500 rpm.
This was driving everyone nuts, we could not crack the problem. I removed the filters and still no good. I was convinced it was a carb issue, so I got the bonnet removed and laid on the front wing and running board so I could watch the carb air slides and someone belted the thing up the road, sure enough the air slides were not going past half way.
After mulling this over a thought occurred to me that the air was rushing past the carbs so hard that maybe they could not pull any air in, I made some crescent shaped covers to fit on the leading edge of the filters to create a negative pressure area in front of the carb venturi, the car went up the road and revved to nearly 6k. This taught me to never be afraid to try anything no matter how daft it seemed.
We have better tech nowadays, you can put a go pro under a bonnet or wing and record all you need.
Short of hiring a wind tunnel I cannot think of another way to find out the airflow over the front end.
I have no complaints regarding my rather over size spoilers, the car is rock steady at 100 mph plus, but I have not taken the car to that speed without them, so who knows, they may be just good rabbit catchers. I guess I need to find a ball of wool !