I shoot my '58 with about 20 grains, then top it off with some grits. That way when I seat the ball it is close to the forcing cone. A little dab of crisco over the ball and it is good to go.
I have never put more than 35 grains in mine, I don't see the point to it anyway. It shoots good with the load I mentioned.
The size of ball you are using will effect things too. While a .451 ball does not seat as well as a .454 ball, I find it is more accurate. But each one of them guns is different. Mine has a 5.5 inch barrel, if your is longer it will change things.
I don't believe you could grenade the cylinder by overloading with BP. Not sure about Pyro or other BP substitutes.
Fun guns to shoot for sure.
You can also get a cylinder for it that will allow you to shoot .45 Colt cowboy ammo. Of course that cylinder costs about as much as the gun did.