My experiance with larger than #9 shot is that as the weight increases, so does the shotshell cups tendancy to crack under recoil. At least, that's what I think is going on.
My latest frankenload with a shot cup consisted of 2 95 gr .380 bullets base to base in the cup with 6.0 gr of Unique pushing it. It chrony'd out at 1081 fps (7.5" RH) and could only be loaded one at a time due to shot cup breakage. But wow what a rabbit load! POA @ about 30' with both 380's striking within an inch of each other. The base first round consistently struck the target like a WC.
I've loaded #6's with pretty good success. Nothing larger besides the 380's.