My Service-Six's devour cast lead bullets and they run below 12 BHN, probably under 11 BHN and they don't lead the bore. My GP variants use the same bullets and they don't lead either. I don't think hard cast is a good idea unless you're running in the very upper ranges of magnum velocities and I'm not convinced it's needed even then. I can't speak to powder coating because I don't powder coat. I don't care for gas checks with revolver bullets and again I don't think they are needed at sane velocities. I understand gas checks on rifle bullets but if you need a gas check on a revolver bullet you may want to look at other factors first such as; bullet size, alloy and lube.
I can highly recommend this reading for the OP:
http://www.lasc.us/Fryxell_Book_Contents.htm