My guess on the leopard flipping is that it's probably a combination of things - too light for your power, a little oat, and the disc has lost some stability. I don't throw my leopards anymore for these same reasons. If you like the leopard get one in better plastic and higher weight that should reduce the flip but you'll still likely need some hyzer given leopards HSS.
As for when to move up - if higher speed discs are not going any further and are less predictable you're not ready for them. It's not just about distance - without snap you'll hit a wall at around 350' regardless of disc speed rating. I bought a bunch of speed 13s early on and they don't go any further than my 9/10s so I don't use them. If the Valk/Archon (9/11) are only giving you at most another 50' over Buzzz/Leopard (4/6) you may not be ready yet.
Oh well, I guess I'll just go throw something.