Your basic premise makes the two loads for all intent and purposes equall in all respects. So the entire question is self answering and moot. Change size of shot all you want they are for all intents and purposes the same just coming out of two different size bores. I will say that it is a heck of a lot easier to get a 12 to do what you want than a 20. In effect you are loading the 12 down more than you are loading the 20 up to max. capicity or performance.
You state a 70% pattern at 40 yards, with the same payload and the same speed. Since you took out differences in patterning the extra amount of shot subject to bore scrub is not an issue as you insist that 70% still hit the pattern target.
Shotstring is not a major factor as the bird will only travel a few inches from the first pellets' arival to the 95% pellet arival. In most statistical analanalsys (real spelling), 95% is about all that you have to worry about. There is always tha golden BB or lucky pellet tha might hit the vital area but most of us are going to rely on 95% chance not a 1:1,000 or 1:1,000,000 chance. So if you look at 95% of the shot load and compare the two different shot strings you would be lucky to find 12" difference. So what does the bird more forward an extra 1/2" as the longer shot string passes by? Big deal. There is not any real difference to a hunter and he will not hit or miss a bird with either load because of which one he is shooting.
Hevishot brings up a more interesting point to me. Round is not a natural, efficient, areodynamic shape. Plane wing as they tend to be tear drop shape because it is more areodynamically efficient. Had lead remain the first choice for hunting loads, and still legal in all places, would some real bright shell maker be selling us loads, with tear drop shaped shot, because it is more efficient and patterns much better?
