A couple of things I'd note:  most of our a/c engines' operating time is at high altitudes and the particulate levels are much lower than closer to the ground, so at +/- 20,000 feet I wouldn't be real concerned about the filter efficiency.  Also they are usually operating at reduced power settings, so even if the filter was restricting the flow and reducing the power by a few hp, it would only be at full throttle.  If the engine is reaching full manifold pressure and rpm there is nothing to be gained.  If one doesn't see a mp/rpm difference between filters, what's the benefit?

On the other hand, during take offs at full throttle, there is a lot more dirt in the air, particularly after having been stirred up by previous a/c taking off and even further by the propeller on ones own a/c. I think this is when efficient filtering of the intake air is most important.  I'd think that in dry areas, e.g., Southern California, Arizona, etc., where most of the unpaved land in the airport environment is bare dirt, the runways and taxiways have a rather high dirt layer (not real visible, but certainly there).  At my home field, KSEE, I will occasionally see clouds of dirt blowing across the airport when the wind kicks up a little.  Even inside a pretty well sealed hangar, I still get an accumulation of dust on everything. All of this dirt is being sucked into the engine air intake, and unless filtered out, into the engine itself.  How much, I don't know, but I'd rather not have any going into my engine.

If you are not already having oil analysis (OA) done on your engine, I would strongly suggest that you do.  I have it done, at least periodically, on all my vehicles and aircraft--besides the L IV, I have a 450 Stearman and a SNJ-5B (Navy T6).  The Jags, aircraft and Triumph motorcycle get an OA at every change.  It is cheap and alerts one to any number of issues before they become problems. Establishing an initial baseline is important, then departures from it become readily apparent.  I'd be happy to help you interpret the results if desired.

I have used Blackstone Labs for over a dozen years now and have been very happy with their service.

The OA would show you whether dirt (silicon) levels in your oil are an issue, and if so, amount of metal from wear. If not, don't worry about it and just keep doing whatever you're doing.  If so, I'd go to a different air filter and carefully examine the entire intake system to insure there are no leaks between the filter and the engine intake (it is not uncommon in cars to find leaks around or aft of the filter).  It will take couple of oil changes to flush all the contaminated oil from the system, so don't expect to see a dramatic improvement on the next OA if you do change things. 

As I say, if dirt levels are not high and wear metals are not elevated above normal for your engine type, don't worry about it.  But if they are, you'll save yourself a lot of money in the long run by correcting the causes.

Bill