I agree, at 4500-5000 RPM (6 cyl) the stock system gets restrictive on the line of 1/8-1/2" of mercury. Some is the paper filter, some is the intake tract. You can get a fractional increase in power by minimizing intake tract restriction and increasing the laminar flow through the tubing. You can change filters put in smoother bending pipes with smoother pipe sidewalls... ETC.

But is it worth a couple hundred bucks to get 4 HP? The results I got on a dyno, didn't even show that much (I kind of wonder if you were running the same conditions between runs, with multiple runs for averaging). Even if cost wasn't much of an issue, have you wondered how a K&N flows more than a paper filter even though it is the same size with less pleating(less flowing surface area)? Larger holes. Larger holes means more dirt passing, which means tearing up the cylinders. When I took my engine out of my jeep at 187K miles, I had been running a K&N for almost 30K miles, washing and oiling it every 5-10K miles. I had .028" of wear on my worst cylinder and .024" on my best. I've torn down engines of friends with 200K on them with stock filters and boxes that are usually on the line of .010-.015", almost half the wear. The stock piston to cyl. wall clearance on most motors is .0015-.0030". Even if the filter did boost your power the amazing 15-30 HP is it worth kill the motor within 190K miles?