[vacuman]

Seen this sort of question since the days we drove our Cortinas with no air filter at all, or at least a little chrome mesh jobbie.

In my view, the easy way to test is to take your filter off and give it a blast down the road. Bet you don't notice any difference - because the factory airbox and filter is not the restricting factor.

My ZTT has a K&N panel filter and 2.5" S/S LongLife custom exhaust. It was in place when I bought the car but I'm very doubtful it's added more than perhaps 1 or 2 bhp. But how often am I likely to be driving at the top of the powerband which, I believe, is over 5,000 revs. I'm not one for racing my ZTT - too many cameras and other idiots on the road. And, these days, your average German 2.0TDI is running nearer to 250bhp than 200 so wouldn't want to embarrass myself trying to beat estate agents away from the lights.