Never ran the stock box but did run a custom box with a 9" k&n with pre-filter also ran 9" k&n with pre filter no box/open air. I did notice a slight and I mean very slight drop in throttle response and top end speed with the box vs the open air filter and I had the same size filter for both applications. I also rode mine through lots of mud and water (no river crossings or anything but wet puddles) with a water proof pre filter from outerwears and had no problems with the open air box eliminator.