as far as hardening & castability more than 2-3% tin is a waste of alloy , it`ll only do so much then the level of benefit goes down fast !
I try to use an alloy around 10-11 bhn for 357mag HPs I take it to 5% tin to tuffen up the bullet without making it brittle so the HP does`nt break off upon opening , well with the tin at 5% I have no problem at 1200-1250 fps
If I need a tuff nonexpanding bullet I usually turn to WW & WD it to get a hard bullet, but still the thought is still there will the bullet shatter if I hit a bone ????