Turbo Diesel intake restriction

[User]

I've been playing around with aero on my delightfully (almost) stock hypermiling rocket. One point I'm investigating is the air intake on these models is via a serpentine and bellmouthed moulded plastic pipe ending in the nearside bumper outside the engine compartment, so it effectively has its own compartment with nothing other than the air intake inside. I've replaced the OEM undertray with something more aero and sturdy as shown in the pictures. The weird bit is that the OEM design appears to have no free flowing intake to this compartment from outside, the original undertray covers the base completely and I couldnt find any significant ways for air to get in.

The experiment involved driving a long distance under controlled conditions with both this undertray segment in place and removed. It was conducted along a stretch of highway without any intersections or significant traffic. I chose a 400km section of it that I could drive back along several days later with near identical conditions (clear skies, same temperatures, engine warmed up over at least 100km, same fuel and luggage load) and just left the cruise control set at the highway speed.

Out (no cover over intake): 3.7 l/100km
Back (undertray returned): 3.7 l/100km

The main variables that come to mind would be the fuel loaded, which was from the same brand but different location (as its summer there should be little difference). Possibly of note is that the FE reading climbed up to 3.7 on the way out after staying on 3.6 most of the way, where as it was stuck hard on 3.7 all the way back.

We're currently thinking that intake restriction on a Turbo Diesel shouldnt be much to worry about which would lead to the conclusion that the aero advantage of this cover are almost insignificant?