if the pouch is pulling away from the stone then it will impart less spin not more.
I'll completely disagree with jaegoor on shaped leather pouches, if they cup the ammo and don't release you haven't made the sling right.
As far as a heavy stiff braided sling goes, over time they WILL deform if not stored correctly.
Balearic type slings need to be stored hanging straight when not in use, otherwise the fibres set in the shape they are stored in and the sling needs to be heated and straightened.
As to the original question, no. if a pouch is narrowing at the point of release it's actually moving into the stone and maintaining contact as long as possible, rather than away from it. This will impart spin and give you control of the stone as long as possible.
But having watched numerous balearic slingers have stones go through their pouches, and out sideways during windup.
I will never trust or use a split pouch sling.
The ones I have are purely for showing people diefferent sling typoes - and in the case of the one I bought from louis, making pistol cracks and making people jump