theory is the open bell mouth lets air flow, gauze can cause disturbed flow, but, main thing is the filter is there for a reason. when we were racing on ex airfields , despite hiring a road sweeper, they were still dusty places. we had a few bikes pick up dirt into the motor while running without filters.
modern superbikes can be tuned using different intake trumpets lengths BUT still keep the air box and filter. As performance is not really a worrry on most classics I go for safety of filter.