Ich habe solche y Slings bisher nur für staff Slings verwendet. Für wasserbomben. Für die Hand finde ich sie nicht gut.

Leder für Balearic Slings.  Kann man machen. Gefällt mir nicht. Der abrieb kommt wenn man kantige Steine verwendet. Das Gewebe zu locker ist. Oder man falsch schießt.  Ich verwende wachs . Funktioniert prima. Der Stein hat einen guten halt. Ich verwende eine Balearic sling etwa 2 jahre. Dann hat sie zig tausend Schuss hinter sich.

went to the range today and i did more tests. I was lead to some conclusions.
Things are grey rather than black and white.

Y-sling

• can handle heavier release knots
  i used two different knots the light one produced whipping sound
  last time i used a heavier knot making it completely silent.
• Most of the forces are on the release cord
  which means it needs  heavier grip on the release knot, it also means you might have little control on the direction of the spin
• never(?) tangles with a stone
  this could be useful on longer slings (>80cm)
• spins: spiral(most of the time)
            discus ! , i could throw disc shaped stones (with or without angle of attack)
            backspin, although the design of the sling seems it will produce backspin easily , i couldn't. probably it was due to twist on the cords which favoured spiral release.

Conventional split pouch
   when changing from Y-sling to the split pouch problems appear.
Y-sling made me used to heavily pinching the release knot leading to late releasing when changing to a split pouch.

• Light release knots/cords smoothen the release, can't handle heavy knots as easily.
  Longer slings might have release problems. Seems it can't be as silent as a Y-sling it would mess with the release.
• Lighter grip
• these happen rarely but still happen : might tangle with the stone in the pouch where the split becomes release cord, or the stone might decide to roll on the whole release cord sometimes even breaking it, producing a very loud whip crack and pulling your arm 
• spins: spiral (with or without angle of attack)
            backspin, i don't suggest this, it raises the chance of the above tanglings happen.
  Has more control on spins

On short slings conventional design wins.the two designs are not far apart, they even have similar reload times .
On longer slings i need to experiment with Y-slings more, conventional design seems to reach its limit of clean releasing around ~90cm.

Note that the conventional design slings i used are linen split pouch with thinner cords than balearics and with less to no tappering on the release cord.

Tapering might help by making it easier to clean release but produces loud whip cracks i don't like...
synthetic materials will probably have different behaviour

as for the clean releasing I can compensate the feel and aiming .Only a high speed camera can help understand if the conventional design introduces any error/unpredictability.

Mersa

For silent throws with a Y-sling you'll need either captive release or a heavy release knot with no loose ends . The simple knot without captive release didn't work. Experiments will help you more than words.

About the other sling design: nothing extraordinary to report ,it's workable and had some close hits, it's more sensitive to tuning than the other designs. As for having a clean release, I don't know. The leather pouch opens and adds drag abruptly. I need to make a better one without leather pouch.