A lot of filter/exhaust performance increase can depend a lot on the car. for instance, most people with my car (using a 2.0 from the 1960's!) can get a 5-20bhp increase with just a K&N and a 4-2-1 and maybe a very light cam upgrade.
Oh and jakg, thats £15......for life. think of how many £5-£10 filters youd buy if you serviced the car regularly compared to a one off payment.