From my personal experience running a vorticity stack on my APE.
Power gains? Hell i didn't notice any, I'm sure I woulde notice more top end by just fasting the day before a ride. The biggest advantage is BLING, when posing just about any moto guy will come over and point and gock at the stupid billet dixie cup looking thing and mutter something like (look that things got a V-stack on it, cool) then they follow up with (is that really a dry clutch?). I should record it sometime its funny, almost like a broken record at this point.
A down side to running with out a filter is you will get more particulates from the air in your carb and motor. If your lucky maybe suck a small bug in. If you rebuild and change things as often as i do then that wont matter to you. If your the type that wants your motor to last as long as possible run a filter.