The transonic thing is one event - pellets obviously weren't designed to "transition" (word from the ELR powder burner world). Projectiles behave differently when they have to "transition", and this can result in unpredictable ballistics. Best to avoid having your pellet transition.
The twist rate thing is interesting. My ballistic knowledge comes from the "powder burner" world, which is not necessarily apples to apples with diabolo pellets. You can't really over spin a rifle bullet - not in a stability sense. A projectile can be overspun to the point where the jacket may separate, or the core becomes deformed, which obviously will have negative impact on ballistics. But generally, it's better to spin faster then slower, as long as you aren't damaging the projectile.
Out of my .22 Taipan Veteran long, I can shoot slugs and 18 grain JSB's to targets out to 273 yards (farthest I've gone so far), with predictable results. The RD Monsters are something else, and I don't think they are getting twisted fast enough to stabilize once they drop below a certain velocity. I'm not sure what happens if a pellet gets "overspun" (if there is such a thing), but I can't help but wonder if a faster twist barrel would help with the RD Monsters.
RD Monsters shoot excellent at ~10 yards for me. Take them to 120+ yards, and they really start flying unpredictably out of my Taipan Veteran. I recently got a power spring from Talon Tunes, which only added ~30 fps, and I just ordered a power plenum to try and get a bit more power out of my Taipan. This past weekend I shot the RD Monsters at 115 yards, and they worked perfectly for that distance. I want to take them out further again, and see where the threshold is for them falling apart now with my power spring, and soon to be power plenum. With our velocities and twist rates, we seem to be operating on the ragged edge of stability with the RD monsters. The farther out they go, and the slower they get, the closer to instability they become. In my experience, at some point they become completely unstable and unpredictable. Some people just haven't shot them far enough to see that instability.
My theory with the RD monsters is that the CG is further back from the head, with that long skirt design. Having that CG further back makes it act more like a traditional rifle bullet. When it gets to a certain velocity with our slower spin rates, it starts to "wallow" in flight, cutting an elliptical path through the air. It wants to rotate around it's center of gravity, which will cause the nose/head to pitch around more, creating an unpredictable trajectory.
I can't help but wonder if a faster twist rate would help. I believe my Taipan is a a 1:17. I would love to try something like a 1:12 barrel to see if stability improves (or even decreases). I'm not sure where to source faster twist barrels for my Taipan Veteran, but I would love to experiment with this if I could.