My experience is that 80' - 85' cars that are close coupled really run best on 080 minimum. I've had some larger cars jump the rails on 072 curves. For whatever reason my GGD P70s don't like 072 curves while my Atlas 85' Comet cars do fine. I think truck centering has something to do with it.
On the other end, 40'-50' freight cars run fine through 054 curves.
Not completely related to this topic, but one of my favorite experiences running Kadee couplers on trains is hearing the locomotive pull the slack action out of the train since the couplers have a small amount of play in the knuckles.
