I haven't tested no helmet, but I've field tested one of the old fairing aero helmets (plastic shell with no foam) and it was slower, for me, than current generation aero helmets. Hard to see how bare head could be faster than a fairing that is squeezed on to the head (so no width increase) and fills in the gap behind.

more aero, looks like. . .

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https://lshtm.academia.edu/MikeCallaghan

http://howtobeswiss.blogspot.ch/

I actually tested this at A2 a few years ago. Unfortunately I do not have the data, but the helmet was more aero (road helmet).

ETA - more aero than my hairstyle, which is short-med length.