I did a quick search for info -
Found a guy who had done some "rigorous" testing in his Miata (coincidently) and compared different filters by hooking up a differential pressure gauge to his inlet manifold.
Now I'm not sure this is a sound as NW_Rob's flow v 15"WG testing of airboxes, but indicative to say the least....
This guy found his stock air box without a filter to even burn off -5"WG at 6500rpm even without a filter.

Other testing (I've subtracted the 5"WG)...
K&N..................-1.3"WG
Paper................-1.8"WG
RacingBeatFoam...-1.5"WG

1psi = 27"WG

So in summary the filter steals about 0.06psi.
I'm very suprised
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To put that into perspective, NW_Robs data at 15"WG for various DCAI and CAI and stock intakes etc. when normalised out to our WOT 6000rpm 90%VE and a 65mm pulley show....
Setup......cfm......airbox......ManiP......rwhp
DCAI......706.....-13"WG.....19.2psi.....330
CAI........702.....-19"WG.....19.0psi.....328
Stock......689.....-36"WG.....18.4psi.....322
So there's obviously a lot more drop in our situation than for the miata's

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Scoops.......
there's a good intro to scoops...here....
Race Car Book
in summary they produce the equation.....
vehicle MPH = engine CFM x 1.64 / scoop area (square inches)
which is the balance point where the scoop goes from being negative prs to positive prs 'cause its capturing more air than the engine can use.
You'd want this around 55mph hey...so at WOT (700cfm) and 55mph the scoop area should be 20 in2

Having said that....
I found this here
The formula for theoretical maximum ram pressure ('velocity head') is:
p = pv²/288g, where
p is the velocity head in psi
Þ is air density in lb./cu. ft.
v is velocity in ft/sec. (88 ft./sec. = 60 mph)
g is acceleration due to gravity, 32.2 ft./sec/sec. on our home planet
So for example, at 150 mph (220 ft./sec.):
p = (0.076 x 220²)/(288 x 32.2) = 0.4 psi (~11 " H2O), or about a 2.7% potential power increase

but when you take that back 50mph.....
you get 0.04psi....so not very much until you're over 100mph (0.2psi) or a coupl'a rwhp