Neat!
That makes me wonder if anyone has ever played with the exit baffle angle. Wondering the effects it might have.
Yes, extensively.
Best results are reported from a next-to-last baffle with aperture as small as practicable/safe, and the exit aperture nearly the "next caliber up" in size. The next-to-last being the smallest aperture in the stack helps contain gas behind it and the larger exit aperture disperses gas in stages, reducing the muzzle pop and hiss effect. Dispersion slits, tap holes in the exit tunnel, and rings of dispersion holes have all been tried to varying degrees of success. One thing is clear, you do not want any parallel "tunnel" in your aperture because the Venturi effect will cause a rapid increase of gas flow, possibly creating a sonic boom pressure wave. All apertures should be internally tapered like a funnel on the back side to encourage the gas to continuously expand.
As for the supersonic bits exiting before (and concurrent with) the bullet, as Bill said that could be powder granules left in the bore from the previous shot, or even carbon chips sloughing off from the inside of the suppressor. The same effect can be observed when thumbing the cork off of a champagne bottle: The gas and liquid drops initially spray out past the cork at a much faster rate than the cork, but as the aperture opens up, the velocity of the stream slows to follow the cork. The initial gas spurt evidently has sufficient volume to propel debris out the end of the can, but interestingly is not observed with the super-sonic bullet. I'm not sure I buy the idea that whole powder granules that large are bypassing the bullet and making it all the way down the bore and out the can, but I can't prove that is NOT happening. Sub-sonic ammunition that will function a 300 BLK AR-15 will be inherently dirty, inefficient, and leave lots of trash in the bore and can.
The double-donut smoke ring is the static gas in the bore/can being compressed ahead of the bullet. If you shoot a few subsonic rounds through a suppressed AR-15, quickly change magazines, and drop the bolt on a fresh round, it will make an impressive smoke ring puff out of the muzzle.