Writers Colonies! Miss Snark has heard such wild tales she almost wishes to take up the pen and apply.
For those not familiar with the concept: Writers retreats or writers colonies like
MacDowell,
Yaddo, and
Hedgebrook are like going to a free hotel for writers. Nothing to do but write all day. Many of them do your meals too, or do meals cooperatively so you don't have to starve. It's also a great place to meet other serious writers. No editors, agents or snarks attend. Here's a link to a whole slew of them
Artists Communities
You bet I pay attention to that in a cover letter. For starters, MacDowell, Yaddo et al are not first come first served. You have to apply -and the competition is pretty stiff. If you get in, that's akin to a literary journal publishing your work: someone read it and thinks you've got something to say in an interesting way.
It would be impossible to quantify "more or less". Sufficient unto the day to say if you won a fellowship to one of these places, it's definitely worth mentioning unless you have six prize winning novels you'd tell me about first.