Like Carl said they WILL give you better performance but only if you tune for them. In other words, if you allow your engine more air but don't rejet, you may end up with too lean a mixture. You rejet and now the mix is back in balance and the bike will respond better. Thats the simple version, it gets more complicated as you do it. You also need less restrictive pipes if you intend to do this. However, with a uni filter, which is what i have, you can alter the degree of filtering and airflow by how much you oil it. if you heavily oil it you will get better filtering than if you go very lightly on the oil. probably about the same as the stock filter. If the reason for the filter is just to have a reusable filter and you don't intend to tune the bike, then that would be the way to go IMO. Just oil it pretty heavily. Of course it's subjective as to how much is a lot, but i think once you get one and oil it you'll get a good idea of whats what. I use the UNI for both reasons. Having a reusable filter has a big advantage even past the savings you get. This way if you think your filter may be getting dirty enough to hamper performance you can find out by cleaning it. With a stock filter you never know whether you really need a new one till you buy one and see if theres any performance of mileage difference. so it's nice to be able to clean it whenever you want and have peace of mind. That plus the savings IMO make them well worth it. I oil mine lightly becauase i don't live in a dusty area at all and i feel it filters just fine like that. and i have tuned the bike with the filter oiled lightly to get as much performance as possible. i also have thunderbike pipes to unrestrict the output.
By the way, i believe the UNI flows MUCH better than stock and i'll tell you why. The foam element itself i'm really not sure how much it differers from stock. The big difference IMO is the "cage". look at the stock filter then look at a UNI. The stock filter is surrounded by a perforated metal can. judging by the spacing of the holes or perforations, only about 1/2 the surface area of the filter is open to air ! now look at a UNI....it's got no can, just a sorta chain link fence looking affair. liiks like chicken wire on a very small scale of course. It probably inhibits maybe 5% of the filter area at most. So in short, if you removed the filter media itself, that being the foam in a UNI or the paper in a stock filter, the flow would probably be almost 1/2 as much with the stocker as the UNI.
