If you are using a .600 ball in a .577 barrel then I think some of the cheese may have slipped off your cracker. Thats dangerously close to causing cracks or blowing a barrel. It just makes no sense.
In terms of round balls in muzzleloaders you take into account your patch thickness and then try to match the bore. That way you get a good gas seal but not so hard to push down that you are swaging the bullet as it goes down the barrel. Of course all of this will vary gun to gun as all sniders are ".577" in theory, they vary up and down depending on wear, corrosion etc.
I would stick to the lands or mabye 1-2 thou over but not much more than that. Forcing the ball down the barrel that hard seems like an inherintly bad idea.
Also it seems to me that mixing cornmeal/cream of wheat with bp would give you an uneven powder burn. Depending on the orientation of the powder and filler. This may not affect accuracy as you say but velocity definately would seem to take jumps and have lows. To each there own i suppose but I have used cornmeal in my snider shells and bp revolvers for a long time and never ha any issues.
Cheers, McLean