Well, you made me think a bit. Yes, the shot payload I am using is over the book limit that came with the gun. No, I do not think it is an over load. After thinking about this for a while, I decided to see how much a CVA shot cup holds. I am using the shot cups Guru told me he had so much luck with that are the CVA brand when I am shooting steel. They are designed for lead, but they protect the barrel from the steel. Anyway, the CVA plastic shotcups hold somewhere between 100 and 110 on the measure. I forget what the factory powder limit is, but I think it is either 90 or 100 grains. That means I am less than 15 grains on the powder measure over on shot with the load I use on turkeys compared to using the CVA cups and a max book load. I guess that means the CVA cup holds more shot than their book allows too! I don't worry about that small difference because I only use that load for turkeys or Geese. The usual load is a lot lighter like yours. I do know that with my light Trapper shotgun, you don't want to shoot the heavy load a lot anyway. It tries to kill on both ends.
If you want to check, use max powder under 100 of shot but try the paper shot cup idea to get you out there a little farther. The thick over powder wads seem the best so far in my gun. If I could get the steel load worked out I would be happy with the gun.
On the other hand, I am ordering a 38 inch 12 guage barrel tomorrow to start building my flintlock shotgun with. I will have about 400 in the gun when I get done. Building it myself is the only way I will ever be able to get one it seems. Maybe this spring I will be using a different gun!