I found my airbox cover lid restrictive on my 78 400, so I rejected the main higher and cut the opening a little bigger larger just below the cover lid under the seat. Now the bike had other big issues such as wobbly crank from when I paid a motorcycle shop to rebuild the engine and from hind site I probably had the timing retarded back then due to my error, so I can't report on a performance increase, but I believe the oem filter was not restrictive. It was properly oiled with the newer foam filter oils at the time.

on my unifilter I cut part of the fold over layer off. You know where it folds over to make two layers

Speaking words of wisdom, let it breathe.