It isn't that there is no co-relation, it's just that the co-relation is not consistent enough for an accurate analysis. There are too many variables to account for, such as ejector, ejector spring, extractor, extractor spring, action spring, reciprocating mass and burn rate of the powder in the ammunition.

For example, if an AR has a weak ejector spring, ejection distance will be short, even if the AR is over gassed. If the AR ejector spring is strong, ejection distant will be greater.

The most common malfunction attributed to over gassing has nothing to do with over gassing at all. That is when an empty fails to eject from the action, preventing the feeding of the next round from the magazine. There will be an empty in the action and the next round will be partially stripped from the magazine, the bolt over riding the base of the fresh round. Many believe this malfunction is due to excessive carrier speeds due to over gassing. They believe the excessive speed has the carrier out-running the magazine. Nothing could be further from the truth. This malfunction is caused by a weak extractor spring and/or an out of spec extractor. Trying to use ejection angle to diagnose how an AR is gassed in this case is the ultimate exercise in futility.

That isn't to say ejection angle and distance should be ignored. The angle and distance of ejection from a well running AR will be consistent. Any change in the ejection pattern should be investigated.