A true velocity stack requires laminar air flow to operate properly. A filter of any type near enough to the bell mouth to cause turbulence, pretty much relegates a velocity stack to the function of an ordinary tube.
If the air is laminar, the reduced cross section from entrance to minimum diameter of the stack compresses the air entering, when the velocity is high. This packs more oxygen into the charge, enabling more fuel to be burned and more power output. A stack by itself will not create a gain. It must be accompanied by a fuel metering adjustment, and some high intake velocities (Mach number).
The stock set up has rubber "stacks" just ahead of the carbs. The "stacks" are fed air from a plenum of filtered air. Filters turbulate air due to their obstructive properties. The plenum allows the air the settle some into a common pool before entering the "stacks". The bell mouthed "stacks" do more to keep the air flow laminar (for the purpose of keeping high and low pressure variations from reaching the fuel jet outlet ports in the carb throat) than to behave as true velocity stack. At least, at the velocities normally encountered here.
The drag racers of old used Velocity stacks to advantage, poking the inlet bells high above the vehicle where scooped air was unfettered body work. They were long, too. 14 inches on some screaming V8 motors.
The short V stacks on MCs probably do more to keep the fuel metering within the carbs consistent than they do to actually compress the inlet air. Of course, if your RPM is high enough to give a significant inlet air Mach numbers...
But, then if you are operating at very high RPMs, that alone is going to shorten your engine life between overhauls. Most likely more than dust inhalation. Speaking of dust inhalation, what kind of dust will your open stack suck in? The soft stuff, or the gritty stuff that sand blasts cylinder walls and erodes valve seats and faces? And, if you are doing frequent overhauls, as racers do, will you care?
If you want the look of stacks without the hassles, you could do what Honda did on the CB700. Faux stacks. See Pic.
Cheers,
