I'd suggest the best way to get more performance from your 250 is to fine-tune the nut between the seat and the handlebar. Learning to ride will make much more difference to the real-world performance than even doubling the horsepower of your 250.
Changing the air filter will (may .. see note about K&N filters) allow you to draw more air, meaning you can burn more fuel whilst maintaining a stoichiometric ratio. If you changed to a high-flow filter and changed the jetting to flow more fuel, and then added a free-flowing exhaust to get all the burnt gasses out you may well a performance boost of about 6 horsepower; this will probably cost you a grand or so in hardware and another $400 in dyno-tuning expenses. Add another couple of hundred if you need the mechanic to fit them for you too.
Regarding the K&N filter, it is generally accepted that these filters don't flow any additional air, but being cleanable/reusable they provide better value for money. For a true high-flow filter think about a BMC race filter.
Why don't you think about investing $400 in a
Superbike School instead.