My two cents worth:
On "slinger's elbow" -- make sure you warm up well before slinging. When I go out in the morning and just start slinging, I get sore. If I take five or ten minutes to warm up and stretch out (I wrote about my procedure
here
), then I don't hurt. Or at least not as much, I still do something wrong sometimes and my muscles complain about that.
On stretchy cords -- I've never tried non-stretchy materials, all mine are nylon braid (either pre-braided courtesy of REI, or I braided it myself). Braided nylon always has some stretch; my 26-incher stretches out another two inches when I pull hard on it. But I always make my cords too long, and use a chain knot to shorten the cords; six inches of chain knot will stretch out another three-quarters inch when i pull hard on it. Between the cord itself, and the extra stretch because of the chain knot, I guess I've got some pretty stretchy slings.
Well, anyway, the point of all this rambling is: if I get a sore arm from slinging, it's got to be my technique, not a too-stiff sling 'cause my slings are NOT stiff.
On where to put the "stretchy" section -- I normally put the chain knots right next to the pocket, so all the extra weight of cord is out at the end of the sling. Once, I tried moving the chain knots so they were right next to the finger loop and the release knot. My theory was that I'd get the rock moving better without that extra weight out there. It's the same reason bicyclists use very very light aluminum rims and tires that are as near to being balloons as they can get away with: they are reducing the mass that undergoes the most acceleration.
All I accomplished was making my head hurt, when the release knot and extra cord whapped me in the side of the head. No extra distance.
I haven't tried putting the chain knots in the middle of the cord. Maybe I should try that...