Carl,
I use nothing but 6s, low brass, 2 3/4" for my 20 gauge with an IM extended Briley Choke...It's $90 a case, kind of a no brainer in my book...For teal season and diver hunting out of layout I use the same load with IC (Skeet) Briley choke system...I have patterned my gun at several increments when I switched my chokes ten years ago...Core pattern still stays near 85% in 30" circle with IM chokes out to 35 yards, 105 roughly pellets out of 140...The difference in high and low brass is the actual foot pounds of knock down power due to speed of shot, high brass...Winchester Expert Low brass will shoot at 1350, Winchester, Estate, Federal or Remington High Brass shoot at 1425 to 1450...I always shoot 2 3/4"...Have not changed in 15 years...
Once the snows start showing up I switch to 2s, high brass, 2 3/4" through mid to late season and shoot a Patternmaster Long Range Unported for the 20....Hunted many season for divers, puddlers and geese with the same load...Now I will not lie when the snows start to linger over head 40 yards and not commiting I switch to Hevi Metal 2s or 4s...The SP10 comes out with a Patternmaster Long Range Unported, for snows only when I start having to snap necks...Same speed as the 20 gauge for shot selection 1325 to 1450s out of the 10, the only difference is roughly 50 more pellets from 20 gauge 2 to 10 gauge 2, swing does not have to change...Of course I pass a lot of birds with my 20 instead of taking long shots...
The one aspect of physics that no one can argue against is that speed is speed what ever gauge you shoot...The only difference is the payload, so if you shoot 1 3/4" payload down to a 3/4" payload 1400 fps is still the same...The core density of your pattern is the only thing that needs to be understood by patterning your gun...Once it is patterned the only other problem is then in the shooters hands...
Regards,
Kristan