Performance Filters
For many years I had the pleasure of running a powersports R&D department - segmented off of a large multi-line dealership located in southern California. We developed high-performance engine kits for both on-road and off-road motorcycles and quads. While I do not have any personal experience with MWR or HE filters, I have extensive experience with both K&N filters and BMC filters. The basic idea of a high performance air filter is simple - less resistance &/or increased surface area = more cubic feet of air flow. It is difficult to increase the surface area of the filter given most stock "dry" element filters are already pleated. So aftermarket filters are left with one option, to decrease resistance. The only feasible method for doing this is to enlarge the "openings" or passages in the filter. A typical "stock" filter will yield about 5 micron passages, whereas a typical K&N filter will have passages as large as 20-25 microns (large enough to see light through when held up). The aftermarket filter companies combat this increased passage-way issue by utilizing sticky oil which is coated onto the element as a means of having particles stick to the filter when passing through. (Note - bikes typically do not utilize Mass Air Flow sensors in their fuel injection systems, MAF sensors are used in all new cars and trucks, and using an over-oiled K&N filter will damage these and cause problems.)
There is no doubt that the stock filter will provide better cleansing of the air entering the engine. If anyone tells you that an aftermarket "high-performance" air filter provides better engine protection then they are full of crap. While these type of filters are used heavily in on-road applications, you will never see an off-road professional race team (involved in long-distance endurance style racing - like SCORE (Baja 500,1000,etc.)) use a K&N filter - why? - because they let too much dirt in!!!
Regarding performance - I have dyno tuned hundreds of motorcycles, and the most power you will receive is maybe 1 - 2 hp at top end. You will receive no benefit in any throttle position other than full throttle because at those TPs the air filter is not the constraint or limiting factor. A couple bikes actually lost power (but I think it was because the owner over-oiled the filter).
So the only real benefit (in my humble opinion) to an aftermarket filter is the "re-usable" aspect. However be careful, most people do not have the patience to clean them properly and will use air compressors or heat guns to speed up the drying process, which damages the filter.
For all practical purposes, unless you are racing and are trying to squeeze every possible once of power out of your engine, protect your investment and stay with a stock filter.
