1/8 ounce shot is not that much extra. What 1 1/8 can do 1 1/4 can only do as well as or slightly better. Your factory 1 1/8, 1200 fps most likely are 10k already. You could look at longshot powder loads to keep pressure down.

Most of us load low pressure to protect old wood not because the barrels are suspect. Weak actions or weak barrels need not be shot. Recoil is our enemy. And when you increase the weight of shot while keeping velocity constant you will increase recoil a lot. If I were trying to increase shot load I'd look to lower velocity slightly. 1100 fps verse 1200 does not sound like that much a difference but it is and anything you hit with it will not know if it came from 1100 or 1200. Dead is dead.