I don't agree.
on one hand, your goal is to get as much air and fuel in as possible in the right ratio, so restriction shouldn't benefit you if it's tuned right, right?
unless some aspect of the carb provides insufficient venturi to draw up enough fuel without additional pressure drop across a filter, which ok sorta maybe, and maybe stock sachs have that to consider, I'd still think tuning could skirt that issue but youre not on a stock carb anyway so, whatever. I can see it more an issue in bigger carbs, I mean you need some negative to have fuel delivery
velocity stack helps there too, more velocity helps. Most airboxes have effectively a velocity stack that extends a little way into the dead air zone which adds to the airbox advantage. but your stack should be totally cool anyway
but there are advantages to an airbox:
it'll make it less sensitive to air changes if your neg pressure is more constantly provided by the airbox restriction vs just external air density; steadier air delivery.
and you can even theoretically net some benefit from resonance similar to the expansion chamber on the pipe
also an airbox can be one of the least restrictive ways to have an air filter. those velocit stack cover filtersare actually pretty restrictive, and also remove the velocity around the lip of the stack and limit it's function (ideally the stack sticks out into the filtered air a lil bit to allow good flow around the lip). Alternately an unfiltered stack just risks increased wear from dust & shit. you can have alotta filter area without much restriction with a proper airbox.
tldr: nah I dont think it needs any backpressure if it's tuned right, assumptions based on sachs needing backpressure for the stock carb are invalid with a different carb, and there are benefits to an airbox especially when getting bigger and when exposed to alotta atmosphere conditions, but generally it isnt critical for your setup.