I've developed a new grid for the model, very slightly larger than the original one. I used a much more highly resolved bathymetry file provided by Steve Colman, which represents a compilation of just about every bathymetric measurement ever made in lake Superior.
About 50% of the grid is masked out, which is a bummer but that's what we get for working in an oddly shaped domain.
I also capped the bottom depth at 20m; I think part of the large ice formation in the previous runs was due to the fact that large swathes of the coastal zone were ~5m deep. In 5m of water, it's easy to generate huge amounts of ice, which is then transported into deeper water, where it is difficult to melt.
One thing to think about: Shallow reaches of the lake as "ice factories"- creating and transporting ice to warmer, deeper parts of the lake.