Let's not run away here and assume I know more than I do ....
My only comment is that I find it hard to believe there is any destabilizing force acting on a bullet such that it will begin to pitch/yaw at33 revolutions per second coming out the muzzle.
1. Hard for me to believe does not mean I am right. I like to think it does, but I've been wrong before, and I will again.
2. I agree with you, there may or may not be good load data out there. To get good consistency, you want the powder to nearly fill the case, so the pressure spikes quickly,a nd the adiabatic shock wave ignites all the powder early, giving it a chance to burn. If you get a low volume fill, as I assume you would using a 308 loaded to subsonic loads, the powder does not ignite as well, and a lot of it goes out the barrel, and back into the gas system. Exactly the reason the 300BLK will run subsonic, while the 762x39 does not do it well is because the 300BLK case is smaller, so it is a higher fill level.