The sails are less like wings and more like big foils, catching as much wind as possible.
Foil is a synonym for wing, so I don't think that's the word you want, but I get what you mean.
I've taken the approach -- and for God's sake take this only for what it's worth and not a bit more -- that below a beam reach the objective of sail trim is to present as large a surface area to the wind as possible. Tighten the outhaul, vang (if equipped, which mine isn't) and cunningham on the main to take out the belly of the sail, and ease the jib until it luffs, then trim it in a bit. If you go low enough, a whisker pole to put the jib/genoa out goosewinged is even better.
Yeah, it is the lack of feedback from the telltales and generally close to zero apparent wind that makes it so hard for me to figure out the right thing to do to optimize.
I don't think you want the belly out of the sail. At least, I have been easing outhaul, cunningham and backstay as I turn downwind, and only keeping enough vang on to hold the boom down without putting tons of tension on the leech.
I have tried running straight downwind with the headsail whiskered out, but there really are a lot of big wind shifts as you sail around this little lake so it is a little stressful trying to avoid a bad crash gybe.