A K&N can add 1-1.5 hp throughout the RPM range and it has been dyno proven time and time again. But you are right that taking the lid off is even better. And even better than that is taking the air box off all together and running the outerware.When i bought my full motoworks exhaust system and fmi i got a velocity filter with it. But i never used it, on checking it over i could see there was actually damage to the material on the filter where dirt could easily pass through. There is no real difference with filters anyway. maybe 0.0001 of a hp gain on one of the cotton types. The real difference is taking the lid of the box. I am just running the stock filter with outerwear cover and it runs fine.