I am only going on the info I had back in the late 60's when I was building and racing. All the testing we did and the dyno figures showed no increase below 80mph.
Also in Australia we had a factory 351 ford with a bug catcher shaker through the hood and it didn't open until around 90mph as ford found it gave no benefit until then.
If you restrict the flow in a venturi effect it becomes a restriction, I found that it was better to have more potential volume of air coming into the air cleaner than was needed, not less.
Don't forget that the air filter itself causes a slowing down of air speed. Also the carby needs an even amount of air from all directions to not impeed the flow down all the carby barrels.