Out of interest would you notice much difference in sound/performance just by upgrading the filter or do you require an induction kit to actually notice anything?
If you fit a K&N filter into the standard housing, it might be a bit "raspier" but thats about it.
If you fit an induction kit, then it's debatable whether there is any advantage other than noise. Some people think it lowers the bhp rather than increasing it.
I've got a Pipercross panel in mine, because it only cost me £5 more on ebay than a new paper element. I took a Pipercross cone off and went back to an airbox with an FRP style cold air feed taken from behind the front bumper, very little difference really.
Might just be me or the fact the my Poomy is older, but I put a K&N panel filter in mine and I seemed to have a slightly better take off. The old paper filter was in good condition when I removed it. Didn't wait to find out about any benefits of fuel economy this time, but in my previous car (Peugeot 206) I saved a fair bit!