It's harder than I normally cast, but it is less than Linotype and just a bit harder than the often used Lyman No 2 Alloy.
Lead: 5 on the Brinell Hardness Scale (BHN)
Wheel Weights: About 12 BHN
Lyman No 2: 15 BHN
Linotype: 22 BHN
For revolvers and the 45 ACP, I have great success with wheel weights (often fluxed and cast as is) with a bit of tin or Linotype added to help the castability of the alloy.
With all that said, I've had pretty good luck with some pretty hard commercially cast bullets like the Oregon Trail bullets in the 45 ACP.
Good luck with your new loading project Old 454. I don't have a lot of experience with Tightgroup powder, but it looks to be right in the ball park for you.
US Army Distinguished Rifleman
Washington State Distinguished Rifleman
NRA Police Distinguished Expert