^^^ QFT.If you don't have full cage, then you shouldn't be doing anything that makes you think you need HANS.
Just to answer your question though, the donuts are meant to support your helmet and to take the strain of g-forces tugging on your helmet during normal driving to reduce fatigue. They are not meant to provide any measurable difference in survivability in the event of a catastrophic impact. While i think they are better than nothing, in the types of accidents that would cause certain death, i don't believe they will prevent any damage.
If anyone has any test data for them, please share. I've never been able to find anything. I suspect that's because they aren't meant to protect in a crash.