If you're talking usual triple K&N's, you can expect another 10-15 HP from switching from the stock exhaust plumbing to triple filters. You will need to a new tune or map for this as you will be introducing way more air, which will require more fuel. I have thought about it doing this for many years, but it does had extra intake noise and tends to move the torque peak up in a bit in RPMs.. Generally, a Rocket with triple filters and a freer flowing exhaust (Jardine.. etc..) will put out dyno numbers in the 150+ range..