Never felt like head guards did anything to be honest... sparring ones cover your cheeks so might stop cuts and protect the skin a bit... as far as impact on the head though... don't feel like they do anything... except make me easier to hit...

With a head guard off I find it much easier to avoid shots than with one on... smaller target and my vision and focus aren't restricted.

Spar with one on because unfortunately I fight with one so it makes sense to get used to it.