You can still buy 38mm *** from tabletennis11.I have 3 38mm balls. I think they are Halex 3 stars from my college days. They are 50 years old. I have played with them but not too much because i don't want to break them. #37 and #38 are correct but people are adaptable and I don't think the new rubbers are that fast. There is a limit to how fast/high the coefficient of restitution can be.
#36, I have a Butterfly video of a chopper and looper were the chopper hits the ball with a spin of 137 rev/sec. The loop is really looping because you can see him "la qiu" or pull ball with his strokes.
#34, yes, any definition would be arbitrary but we have not defined optimal spin to speed so we don't know what fraction of optimal is still a loop. The other problem is that we can't measure accurately with our eyes.I still say that most people on this forum only think they are looping because the Magnus effect is small and the arc is small because they hit the ball too fast.