FMF makes a product called a ram valve. Basicly a reed cage stuffer that is supposed to help keep velocity up in the intake at low throttle settings.Boyeson makes the rad valve which is its own reed cage assembly which is supposed to help straighten out the airflow and smooth out the turbulence that is inhereted with the offset carb becasue of the single shock bike as MR.Clean stated.Its purpose it too make sure that both sides of your cylinder gets even distribution of fresh air fuel.
I have a Rad valve and I am not so sure if I actually have more power any where but the bike just seems a bit crisper and and more responsive to jetting changes. It just seems easier to tune to me compared to stock and helps the motor run the way it is supposed to, plus you get a slight improvement in fuel milage since you do have to lean out the carb a decent amount when you first bolt it on. I have no comparison to the other makes though.