Been using cupped pouches for a while, but have recently tried a split leather pouch and... wow. I just "feel" like they are more accurate and release so cleanly! I have one problem though, I love using 60 gram, 1 inch steel balls I buy in bulk and recover after slinging into a dirt pile. I am trying to find a pouch that will open completely and give me the sameclean release feel as the split pouch. Will just shrinking the split pouch itself to the point the ball won't fit through the gap work, or am I doomed to not enjoy that clean release feel unless I am slinging rocks or elongated projectiles?

When you say split leather pouch, do you mean two overlapped narrow straps of leather(first photo), or a single wide strap with a slit cut into it(second photo)?

I'm a big fan of the first myself, but can't say I have much experience with the second.

I've got a number of overlapped leather pouch slings with about 7" straps making up the pouch - these deal well with anything from a baseball(more comfortable with a tennis ball though) down to a golf ball without any issues. But a golf ball is almost double the diameter of your steel ball, and only 45g, so quite a bit less dense.

A shorter length pouch would help as it would require more weight to force open the gap in the straps, although I haven't tested anything quite so dense and smooth as a steel ball so I'm not sure how short you'd need to go. You could test it before cutting the straps shorter by just pinching the straps together at different lengths with your fingers and see if the ball falls through. Not a perfect test, but should get you in the right ballpark.

Another option is you could sew a small stitch to prevent the two straps from opening - don't tighten it down in a way that it pulls the straps together more than they are naturally, but just enough that it keeps it from opening further. This would also work for the slit-leather pouch if that's the style you're using.

You could also consider something like one of my spaghetti-sling builds - both of which can have their pouches adjusted to very specific ammo sizes and are the most secure pouch I've ever used(when sized properly) and has the least interference on release of anything I've used. Although the downside is then the sling is specialized only for that type of ammo and really can't be used for anything else unless it is a very similar size/shape.

Nearly-Knotless with spliced dyneema
Or Sheet Bend pouch that works with any type of cord and is a little easier to put together since you don't need to learn how to splice dyneema.

Good luck! Round ammo is definitely the most difficult to design for - especially as you get smaller than a golf ball.