Sikh units dont seem to have suffered particularly for not wearing the helmet, though I think it would be almost impossible to determine either way...
Typically what happened during WW1 when the Sharpnel helmet was introduced as a result of trench warfare, was a large increase of survivors with head injuries reaching dressing stations, these men would likely have been killed instantly otherwise....
It should be noted though that the main reason for producing the helmets had in fact disapeared by WW2 as true shrapnel for which the helmets were designed(proof against a pistol bullet) was no longer in use to any degree and Slinters from HE shells generally have a much higher velocity:

https://en.wikipedia.org/wiki/Shrapnel_shell#World_War_II_era

http://www.militarysunhelmets.com/2013/turbans-of-the-indian-army

I like the figures, but not the slung rifles, which unfortunately are all too fat for my taste.

Would prefer the helmets to be left off too. Regards, Pa