<snip>from what I can tell the only reason a higher flow filter would give better mileage is if <snip>
The guys who designed and built the Insight and the Prius are smart guys, right? Duking it out to get the best mileage possible for their company's flagship hybrid vehicle. And neither the Prius nor the Insight has an exceptional air filter in terms of size, construction, media, etc. If an air filter could give even 1% in increased fuel economy, then surely the Prius or Insight would have something out of the ordinary, but neither one does.
I'm of the opinion that automakers are always compromising. The Prius doesn't have rear wheel skirts - why?
There are almost always other considerations in play. Look at tire pressure - many Prius / Insight eco-drivers run well over 50 lbs, but the factory spec is much lower. Despite the fact that 50+ returns better FE, the factory recommends lower, taking ride comfort, wet/gravel handling, tire life issues into consideration when it issues its numbers.
Perhaps filter choice is a rare cut & dry selection, but there could be other issues. EG: at fractional throttle openings, the K&N offers no FE gain over my stock paper filter, but perhaps it really does perform better in high-flow situations, giving me more power (and better FE while delivering it) on those rare occasions I open the throttle wide). The trade off may be filtering ability.
For the Integra, the cheapest paper filter is costly, as it is large, cone-style filter, with a heavy-duty plastic tube connector:
I placed a K&N OEM Cone replacement filter on the car about 5 years ago, and my only cost has been another can of oil spray and cleaning kit. Inspection upstream of the tube has been clean thus far (40,000 miles at least) unless you go overboard on the oil (only once ) The element looks a bit weak from where the hot-air intake blasted on a concentrated area, but the oil fills the gaps.
I think stock is identical (unless WOT is concerned) ...but if you pay a little more up-front for a K&N, you can clean and re-use the filter. Less waste.
If you didn't care about keeping the filter, could you cover part of it in tape in order to create different restriction ratios for your testing?
I have a K&N too (sucka!) and it looks like it would still survive taping and de-taping because of the plastic web-grid.
I'm keeping the K&N because of the cleaning and the "appearance" that it can take the greater WAI temperatures I am generating. Maybe it deserves a cleaning this weekend (just in time for allergy season!).