I've got no statistics, but what I can tell you right now is that method is highly unreliable. Your survival instinct will kick in during your sleep, and you're left with brain damage or become a vegetable. I haven't been in SS long, but I can tell that no one is using it because it's that unreliable. It could work in theory, but it's practically impossible. I've seen pictures from PPH where elderly dies using exit bag, but they were the lucky ones. Forget this method and find something else.
When people choose asphyxiation method, they choose monoxide poisoning instead because they just sleep on it and die on the following day, and that's if things are going well. I believe that they have nothing to tie around their neck, but they are confined in a small space. I read months ago in some post that a SS member CTB by monoxide poisoning at the forest in his camping tent with some kind of machine that produces monoxide. I don't remember who, but he may have died, or God forbid, be in permanent vegetative state. I'll search into it someday.
I digress. Anyway, sorry that I don't have the data. I know that you're seeking a "holy grail" of quick and painless death. The only way that's possible is getting a high quality N or SN.