A certain amount of experience on wild pheasants (and I've also done a couple "continental" shoots, plus have shot the real driven birds in Scotland) leads me to believe that there's not much difference, in terms of vulnerability to shot, on a crossing bird versus an incomer. Vitals are still significantly more exposed than they are on the going-away birds you'll often see when walking them up with dogs. You don't have to drive shot through the backbone and the gizzard, and the head is still fully exposed on a crosser. Shooting wild pheasants over my dogs, I'll take a crack at a crossing bird farther out than I will one that's showing me nothing but tail end. Crossers are harder to hit, but significantly easier to kill.
For a very long time, until a mania for really "tall" birds came into fashion, the Brits seldom shot anything more than 1 1/16 oz, typically Brit 6's (which would be about 6 2/3, American shot size). I've used those loads to great effect, over dogs, on wild roosters.
