In pursuit of lower recoil I’ve done some fairly extensive tests with loads lower than 28g and came to the conclusion that “on average” you give away between 2-4 clays when dropping from one to the other, shot size evaluations can be very very difficult to quantify with any kind of authority but logic says the smaller shot size of 8 makes a lot of sense since it patently retains enough energy to break edge on clays at silly range and is undeniably going to throw more pellets at the target.
That’s not to say that open chokes and 21g won’t break long distance clays, just that at the end of the day if you do enough volume shooting then you can’t deny more pellets are going to make a difference. I’ve yet to see a competitive shooter use less than what they’re allowed.