A quick update, since I'm here:
"Payday" has garnered about 2,000 downloads since I made it free last week, and while it's starting to tail off, it's still going at a decent rate. Haven't done any promo on it or anything. I do think that the five-star from the Top 500 reviewer gave it a boost on Amazon, but I've also seen a bump on Kobo, Barnes & Noble, and Apple Books, so it's probably also just the fact that it's a timely story.
The other stories haven't performed nearly as well, but they are getting daily downloads too.
Has it led to a boost in sales? Well, I've sold one copy of my short story collection so far, which is probably one more than I would have sold otherwise—and since at $4.99 that's the equivalent of selling 10 stories (which is also probably 10 more stories than I would have otherwise sold), that isn't nothing. But besides sales, I've also seen a small boost in four- and five-star reviews and in organic newsletter signups. Again, that isn't nothing.
Is this free short story thing a quick path to riches, fame, and glory? Probably not. But it does seem to be worth it so far, especially considering how little those stories were doing for me at any other price point.