This one from Schnitz... How do these differ from the Short Rubber Velocity Stacks? - The Factory Pro Velocity Stacks work best with stock engines. These help to increase power in the mid/upper RPM levels and work very well for both street and strip applications. The short rubber stacks help to move the power curve up and typical gains are above 10,000 RPM. These work best for bikes with extended rev limiters, piston kits or nitrous kits to help move the power curve up in RPM.
Sounds like someone needs to test these things in the real world