do you think this will work with sour cherries (ripe of course)?

Yes, it does. I have a pie cherry tree, which produces the "sour" sort of fruit. This is one of the first things we make when the cherries ripen. I do opt to stone them, though, because I don't care for chomping on the pits. I'd taste the cherries you plan to use first, and decide if the sugar needs adjusting- but if they are at peak ripeness, it's not usually necessary. At least not with the ones I grow.

I think it would be marvelous with sour cherries. The tartness would make the clafoutis' sweetness even better.