how can you get more power? start by taking them off lol.
seriously tho, as said above by mr bognor, the K&N 57i is an open filter which will suck in hot air and loose you power, especially in summer. most think they gain them power because of the noise but when they go back to standard airbox/filter they feel its faster.
to get more power...
put your standard airbox back on with a pannel filter of your choice (K&N, Pipercross and ITG do them to name a few, around £30)
get a custom cat back stainless steel exhaust on the car with a decat or if you cant be bothered with mot trouble then a sports cat, but expect to pay arround £400 for a catback with sports cat)
then get the ECU remaped to take the mods into account
you can do a lot more than that but it all starts to get rather expensive and not worth it