Installed an Ace Velocity Stack (from Guaire) in air box with previous Ace filter kit (don't think stack would fit with stock or KN filter). Without fuel and exhaust mods I don't expect to see any change in performance.
The stack is a tuning device. The length is added the the inlet tract length to set harmonic wave action activity in a certain rpm range. This wave action enhances ramming effects for cylinder filling. In this case it mainly affects the upper rpm range around 5000 and above.
Also, wave action from a stack is only really working right with the throttle wide open. On a partial throttle, the throttle blade breaks up the moving wavefront, stopping most of the effects. It's a WFO performance option which comes into play when you are really riding hard with the throttle twisted open a lot, or going for top speed.
It isn't something which will be noticed when tooling around the lake at 30mph, or going to the supermarket in town.