What's missing is length, a small opening with a flared end will flow pretty good, where a long tube won't. Even then, something like a 460 EFI with the two small flared tubes picks up 25 horses or so with a few seconds work and a hacksaw. The factory stuff has a lot of things that aren't there for power, noise suppression and smog regs are tops on their list.
Lots of air cleaners had small openings, I never saw one that didn't benefit from cutting more holes or flipping the lid over.