Joe,
I would not worry about it because you won't have bridging with number 9s. The wad in those should be extra thick and you can verify that before using. My hand loads of one ounce of #9 TSS call for a 28 wad inside the 20 so there is double protection and the shot fits way down in the shot cup so there are no pellets to contact the barrel.
I patterned both a VH 20 and a Sterlingworth 20. Made a shot trap, and recovered 90% of the shot. Would have recovered it all if I had made the opening bigger. Both guns were full and xtra full in the left barrel. At 40 no turkey would make it through, and possibly at 50 as well but try 40 and inside as my goal.
Amazingly they put 1 5/8 ounces of TSS in the 3" shells due to the density/weight of the shot.
https://parkerguns.org/forums/showthread.php?t=23838