Another thing possibly to watch out for is not just the filter area but the box itself. There are more than a few cars out there benefit from more holes being drilled to increase the inlet to the filter. Very easy to sort out usually and worth being aware of.
That enters the realms of ram pipes then. The original Rover cylinder airbox has a noticeably small diameter ram pipe; I know from my reading what the purpose of this is. The VW box is larger but not necessarily more efficient; I will take a look at that.
