I was listening in to a conversation a few weeks ago between some lads that owned tuning Co's that specialised in Ford and Renault and their general feeling was that it is totally hit and miss and its a hell of a lot of money to spend and run the risk of only getting some nice noise or even at worse case power loss, one lad swore by panel filters and said on some cars even the oem ones performed better, on his 400bhp Cosworth he has a performance panel filter and has drilled some holes in the air box for better flow.