FWIW I experimented a lot with the air box and found that the flow thru the box can dictate performance in both mpg's and higher end loads. Truck = 2005 Dodge Ram 2500 diesel with a bunch of mods.
Cheap air box V1:
An $8 trailer butterfly vent mounted on the bottom of the air box. Closed got me no vent performance as expected, open got me higher load performance.
Not so Cheap air box V2
Used a 3" electric exhaust cutout with a cab mounted switch. This got me on-demand valve openings, no more crawling under the truck to open the $8 vent.
Not so Cheap air box V3
Used a boost controlled 3" exhaust cutout for automatic control based on boost signal. Valve opens at aprox 5~6 psi of boost and boost is capped at 10 psi with a boost control valve so actuator does not see high boost. Valve performance is regulated with a simple bleed valve shown in the 2nd pic (lower brass valve). The goal was to keep the valve closed under low boost conditions and open under load. What this got me was a mod that jumped up into the top 5 if not top 3 for my truck. I have also been rewarded with 12~15 tanks of 23.0 mpg or better (23.6 peak) last summer and have already recorded a 23.0x mpg tank this year in less then ideal conditions. Previous to this mod mid to upper 22's were the max mpgs over the previous 3 summers.
Not so Cheap air box V4
This will be the same as V3 but with a 3.5" boost controlled exhaust cutout. Will evaluate which one I keep in there.