Have you tried Pipercross OEM-style replacement filters? These are the different-density foam style filters but fit in the airtubes.
Pipercross part number MPX-038 and about £18 a pair from
http://www.Demon-Tweeks.co.uk/ Should give increased flow rates through better foam design, but keep the large volume of clean air available in the airbox.
Would be interesting how these rate against under-tank filters.