[Tesla]

Think I've said it before, cold air intake is generally preffered for diesels, but that is within reason, depending on the engine tune & fueling map.
I think ideally an intake that could maintain a constant temp year around would be best, and then adjust the tuning, intake, exhaust etc. around this particular temp range.
Getting more air into a diesel increases efficiency until the point is crossed where pumping losses start to pull the power curve back down.
Warming air up from below 0C will definately improve combustion efficiency, but once you get over 50C, then you will start to have a significant effect on the actual air mass going to the cylinder and this will have a major effect on both efficiency as well as power.
Absolute Zero is -273C, so if the same pressure & volume is maintained, going from 0-50C will be a decrease of about 18% air mass actually entering the cylinder.