K&N flow very well for a filter and are a great filter, but for all round drivability you want to run as long a ram tube as you can. For max power a short ram tube will suffice, but the bigger the cam, the more likely the carb is to spit out a short tube.... (But one of my cars runs a 520X and does not spit out with the longer ram tubes!)

Sterling - Have a look at what 90% of rally cars running sidedrafts - see what they run... If you look at what the "competition cars" are running vs the "show / road" cars, you will find a similar answer...
On paper the K&Ns wins and looks much nicer (hence used on 90% of the shiny Escorts), but in the real world (where the cars performance is key and the paint not so shiney), socks do a better all round job (allowing the use of longer ram tubes as the biggest benefit) and hence a more ballanced real world performance.

Nick you need to go and try a set of socks on a flowbench, what you will find is that they are terrible! they have the worst flow of most things I have tried!
The 75mm tall K&n's flow enough to run any good 200-220hp 4 cylinder engine, and will make better hp than a single filter like a single K&N or a sock due to surface area and less turbulance around the throat.
These days you will hardly see any socks on decents engines as most people have worked out how bad they flow. The other thing is the the old thory of needing a longer ram tube has gone by the way side as it's more a case of ports vs cam timing vs exhaust flow as a 520x camshaft is a dog at the best of times and usually requires a bit of trial and error on a dyno dialling in the cam to minimise the reversion due mainly to the poor exhaust systems people use.
If you have a decent head, cam and exhaust it wont matter what length ram tube you use as it wont effect the spread of torque as most of my pinto engines end up with around a 30mm long ram tube with 45-48's and always make better power on a cotton type air filter like a K&N than any of the foam filters as my old pinto was the perfect example as it would suck the socks into the throats when standing on the trottle at 4000rpm with only the auxiliary venturi stopping it going further and that was with no oil at all. We then ran it on no filters and then onto the larger single element K&N which made the same top end hp as open intakes with 3 more hp at 4000rpm!
With the Pipercross filter be careful with them as they are quite ordinary as a filter medium as I have on on my YB but there it always a bit of crud on the backs of the butterflies which you never see with a K&N but it's the only thing I can fit in the engine bay
Bang wfor your buck the best thing to buy is a pair of K&N's and a correct set of ram tubes which have at least he opening oriface sized distance between the end of the tube and the underside of the filter plate

Danny