Bladesmith's input about a test which showed that shot at the shot column's bottom did not reach the pattern circle (which I assume to be our standard 30" circle at 40 yards) shows that my question was ill-reasoned. What I was theorizing was that shot from a 20 gauge - one with a comparatively longer shot column to its diameter than that of a 12 gauge - would encounter more barrel scrub and more compression to its bottom shot - - sufficient enough to slow a higher proportion of them down even though they'd still strike within the 30" circle. My little mental model, then, had them arriving late, but still at the right station. In the tests that Paul notes, however, they miss the station entirely.
The other part of the matter deals with the possible negative consequences of late arrival. Would shot be strung out enough so that there would be a practical lessening of hits to a "big fat duck" at 30 mph/40 yards? Its clear the concensus here is that it wouldn't. In fact, what lateral spreading that does go on is better expressed in inches, rather than feet.
This post reflects higher quality responses than the quality of my original question. I really appreciate the good thought put forward in the former.
