If you do a google seach you'll find a bunch of dynos of various cars (I never found one for a svt focus, but I imagine you could get some idea of what to expect). On the graphs I've seen, there was absolutely no gain (sometimes even consistent loses) over a clean paper filter. K&N's are nice because you never have to replace them, just clean them. That's the only real advantage (unless your paper filter is really dirty, then the K&N will make a difference). In some cars (like on LT1 camaros), they can be a bad idea because some of the oil from the filter can trip the MAF sensor and cause the CEL to come on.