I have always thought that some small “pod” type filters are possibly as equally restrictive compared a standard air box (without access to a flow bench an owner has no real way of knowing).
When I bought my 350 I had plans to fit a high level exhaust which required removal of the air box and the rubber connecting hoses were all perished in any case, so instead of buying new I used a pod filter on the Mikcarb VM24.
I “up-jetted” the bike to compensate. Not having ridden the bike with the standard air box I can’t say if it would have made any difference.
However, due to a fuel feed problem with the VM24 I bought and fitted a “Wassell” 26mm concentric carb. It came with a short, screw on intake bellmouth. Rather than remove that in order to fit a pod filter, I retained it and fitted a larger, oiled foam “pit bike” filter I had in stock (originally purchased for a three cylinder Suzuki G10A engine in my trials car but not used as I fitted a supercharger rather than throttle bodies).
(These days I will never run an engine without an air filter, so I wasn’t happy to run with just an open bellmouth).
The filter sits right over the bellmouth so it doesn’t interfere with intake flow. The theory is that the end clearance between the intake and the inside face of the filter should be at least equal to the intake diameter and in this case that parameter is satisfied.
All I can say is that the improvement in performance was better than expected. I therefore believe the bellmouth has been very beneficial.