Jonathan --
Thanks very much for your very helpful reply. It's given me several things to think about.
I'm sorry if I gave the impression that I expected to be able to underexpose by any amount I pleased and still get good results. I agree that that would be pretty unreasonable. I had chosen to underexpose at low ISO's (100 whenever I could) and use exposure compensation in ACR, rather than expose correctly (or closer to correctly) at higher ISO's, because I had been told by someone I had reason to trust that I would get lower total noise that way.
I appreciate the evaluation of lenses for stage photography, too. Most of my theater photography has been done with a 70-200 2.8L, so I have a start, perhaps, on suitable glass.
A second camera body was definitely on the horizon, anyway, so I'll take your advice on the 1D MkII if circumstances permit. In any event, I'll now be very careful to check noise levels and banding in any model I'm considering. And in any specific unit, too; based on what I know so far about the banding phenomenon on the 1Ds, it's clear that it's different on different units in which it's present, while being wholly absent from others.
Thanks,
Bob