For my .44 mag. I favored using a hard cast Keith style SWC even at high velocities. Never had a leading problem. My bullets were bought from a now closed company in Arkansas called Oliver's Bullets. They were the best cast bullets I ever used and were quite reasonably priced. Wish I knew his alloy formula, I would cast my own but haven't been able to duplicate.
Yes, softer bullets are "no-no's" for high velocity.