Why do I think that this makes less than zero sense? I haven't run the numbers, but I'm quite certain that a standard (pr even +P) 9mm round striking someone wearing body armor is NOT going to knock the person down. If we REDUCE the velocity by 80 percent, it's barely going to sting, even to a person who isn't wearing body armor.
In round numbers, let's say a 9mm round travels at 1200 fps. Reduce that by 80 percent and we get 240 fps. That's slower than most BB guns, by a considerable margin.
Don't ... think ... so.
The projectile is slower but significantly higher in mass. I'm not saying it WILL knock someone down, but it will transfer quite a bit of energy to the person. More energy, in fact, than some 9mm hits. (9mm often goes through a person, and the energy retained by that projectile isn't imparted to the target.)
Add to that the "wind got knocked out of" effect of being hit in the chest with a 1.5" sphere of meta land it could very will make many people sit down where a 9mm hole through them wouldn't. (at least right away)
Not defending this as a great idea, but the concept is the same as the 12ga beanbag rounds. I am entertained that the video's of use all show the LEO screaming "LESS LETHAL" several times in what, I assume, is an attempt to forestall the sympathetic fusillade that often accompanies an officer shooting something.