Fitting a K&N filter won't improve BHP but you should get a small but noticeable improvement in performance. My son did (on his 300 TDi Range Rover) and I did on my TD5 Disco. Whether it's worth the additional cost is debatable. You can also add a friction reducer to the engine (we both use Activ8 and, again, have noticed improvements. Another cheap way is to disconnect your viscus fan. Under probably 95% of operating conditions this is not necessary and is certainly using up precious bhp.