Since I started using it, my AHI's have been sub-10 every night (which was pretty rare, compared to before), including a 4.83 (which is close enough to normal for me).
What amazes me is how you pretty much *don't* see any Google hits that suggest a cervical collar when you search for "CPAP mouth leaks" (or anything similar). Most of the search hits have to do with fixing the mask...or maybe using a chin strap. None of the medical websites talk about even the possibility that proper pillow/neck position, combined with something that helps keep the mouth closed, might actually be a solution that helps people, like me, for whom CPAP therapy was failing badly (usually > 100 events a night...I once took a 2 hour nap for which my AHI was 29). Neither my pulmonologist, nor my respiratory therapist at the CPAP supply place even tried suggesting a cervical collar or similar approach to keeping my neck straight and mouth closed.
But the simple truth seems to be that the cervical collar is actually making my CPAP therapy work. Perhaps for the first time in six years on CPAP.
It might even explain why my problems didn't show up on my sleep studies: the beds and pillow combinations they use there aren't quite the same as the ones I use at home. Combined with the weirdness of the wiring, and a split-night study (like my last one), the sleep tech might never have seen clustering cycles of events that happened for me at home. She certainly wouldn't have seen how things got worse (at home) when my machine was changed to autoset and my obstruction events led (predictably) to even worse mouth-open, chin down clusters of events.
It's still too early to know how the collar might work in the long run for me, but I'm guessing that, since I'm no longer waking up several times a night with a dry mouth and my mask leaking like a hurricane, it's doing some good.