I agree to some extent AS.
"If" such a sling is more forgiving it would be a nice plus. I also think its likely that if any sling were to be more forgiving it would not surprise me if it was one that remains static as much as possible within the throw.
That being said I think when discussing large differences in weight and shape we need to be careful how much credit we give to the sling. Truly large differences in shape are beyond any slings ability to help. Too many things happen after leaving the sling for what happens in the sling to matter.
Obviously the poorly shaped ammos weight plays a big role in this but things get really complicated because its, imo, proportional to the speed being slung and how poorly its shaped.
Heavier ammo thats poorly shaped shouldnt plane as much in the air because its naturally harder to sling very fast and it should be more resistant to change in direction anyways. So it sort of helps itself in that regard. So we get all sorts of variables up to and including the slingers own strength.
Then you have the problem of ammo rolling out of the sling when said ammo being poorly shaped may take a fraction of a second longer than another stone. Such differences make a big difference in point of impact but are almost impossible to adjust for during the throw. Nor could one even know in what way they would have to adjust until after the stone shows us after which it would be too late. Was it you, or perhaps JudoP who defined it as a "zero feedback loop"? Its a perfect way to look at it imo. Let me add my own term- "Shrodingers Slingstone".
Mostly though from what ive seen and maybe this is just me, but if a stone is relatively heavy and relative round its going to be , wait for it(!), relatively accurate.
And really, relatively accurate regardless of whether im using a paracord sling with a wide grip or a stiff balearic sling. On the other hand if its poorly shaped Im not going to be accurate with it in comparison, regardless of sling type.
These are my thoughts from my own experiences. Your mileage may vary.