I initially ran a K&N filter on my 450 after I bought the car. I don't know what the exact bhp increase was but there was a noticeable feel in the torque.
I recently did a major transformation (roadster turbo, janspeed exhaust, venom induction and remap) and am no longer in need of it. If you are interested I can sell you the one I have for $20. It was used for about 5000 miles.
I also have a brabus sport exhaust available if interested. Adds 5bhp. Used for about 5000 miles as well. More power, a bit louder than the stock exhaust but sound nice.