Went down as far as 25 grains of AR 2205, lots of sooty, greasy cases (which I hate), and a pretty low velocity with it, about 1250 fps from memory.
I've found that with the cast bullet loads I have worked with, a fast/medium powder works better than the slower powders, as they tend to reach a higher peak pressure quicker, which seems to seal off the cases much better.
Most of my cast loads in all calibres run from about 1400 fps to 2000 fps, seems I can usually manage to get a load that shoots within that range somewhere, nearly always around the 1600 fps mark.

DC