Diesel intake restriction reduces efficiency.

[Tesla]

Just came across this study:
http://www.icrepq.com/ICREPQ'09/504-mamat.pdf

Conclusion
The results show that the pressure drop in intake
manifold gives negative impact not only to the engine
efficiency and power density, but also in terms of engineout
emissions.

[H-Man]

Cold Air and Ram Air - EcoModder
Seeing how more air helps FE for diesels, the opposite should be true for less air.