Excellent post Mike. you did however omit one important point. In reality the 7/8os load in a 20ga is a lighter load than the 3/4oz load in the 28. Proportionate bore area (.615²/.550²) shows the 20 capable of handling a 25% heavier load in proportion to its bore. This would mean in reality if both were loaded to the same density you would have 28@3/4oz = 20@15/16oz or 28@1oz = 20@1¼oz. This would translate into an effective range increase of 12% rather than 8% if each is loaded to the same level.
It is also noted in the case of the 1¼oz 20 ga load this is put up in the 3" hull giving more room for better, more effecient wadding.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra