This became very apparent a few years ago when I was playing around with a caliber called the .300 Whisper. It shoots .308 rifle bullets at just below the speed of sound. I was almost shocked to find a 200 grain Sierra Matchking was completly stable and accurate out of 1:10 twist at these speeds. A 1:8 twist was good for the 240 Sierra Matchking. Not much different than much bigger calibers driving these bullets almost three times as fast.
I'm guessing that the reason your 200 grain MK was completely stable at that velocity with only a 1:10" twist was because of the different aerodynamic forces experienced by a subsonic projectile, perticularly as it pertains to the bullets center of gravity (CG) versus its center of pressure. If you know anything about supersonic flight you'll know that as an aircraft passes through the sound barrier the center of pressure shifts dramatically forward, which can be a very dangerous condition for an aircraft, as the CG can suddenly be behind the CP, which is a VERY unstable condition. I would think that a similar condition exists with the 300 Whisper. If you pushed the same bullet to just over supersonic velocity you'd probably find that the bullet was very unstable. Aerodynamics at subsonic velocities is too different to make a comparison to normally supersonic projectiles.
Mike