The HKS filters are perfectly good that filtration test was just a marketing stunt, still holding strong though good work Apexi! The HKS units will filter just as good as the other units, if you think about the properties of the filters then the Blitz stainless mesh filters are probably the worst but they will stop most of the crap.
A filter is a filter at the end of the day, there very simply made and there's not much in it. Filter location is most important, if your installing one in the engine bay or near a hot source (like where the stock air box is) then your wasting time.
There designed to be in a direct source of cold air, ideally outside the car or in direct flow away from heat sources. Placing one where the old airbox used to be will only create flat spots. It will be better at night but hot days (or should I say mild for the UK) will render the car slower than before.
HKS filters are good as there compact and fit in most tight spaces, I've been using one for nearly five years (not the same one). Don't believe all the hype you here
T