I know what you mean. Most of the folks with 20" HBAR profile barrels or longer have no problem with the 1:9 twist shooting 75-77 gr pills. The guys with the 14.5 inch M4 profile barrels with a 1:9 twist can't get them to shoot for love or money.
Goes back to your point about velocity and a slow twist giving the same RPM stabilization as a lower velocity and a faster twist. Obviously there are barrel lengths between the M4 length and M16 length (16 inch middies and 18 inch SPR profiles come to mind) that are going to need to be tested by their owner to see if they can shoot the heavy pills.
On the internet it is easy for armchair quarterbacks to point out stability formulas, but the only real answer is for someone to go out and shoot to see how their rifle performs. There are some good formulas though, but a lot of the "marginal" stability twists have been winning matches for years.
Machine guns are awesome until you have to carry one.