I've added cold air intakes to most of my vehicles. Prior 3 are: 05 Ford F53, 11 Ford F53 and now a '15 Freightliner DP XCS 380 hp chassis. Every time it helps especially in headwinds. MPG and Hp improves over stock.
The latest is probably the best. Just got back from CA to TX via I-40. Several 7000' climbs, many hills and cold air. The vac. gauge for testing when the air cleaner needs changing was 2 points (lines) lower than before installing a scoop (used a large commercial dryer vent) to force more cold air into the intake. Got 8.9 mpg same as going to CA with less hills and warmer temps. I expected a full MPG less going I-40.
I do allow for rain drainage so the paper air filters do not get wet.
2015 Fleetwood Discovery 40E on a Freightliner XCS chassis with a Cummins ISL9 pulling 1 and/or 2 motorcycles, '07 Honda Accord OR a 17' Runabout Boat.