Pure wheel weights are about 12BHN. Water drop them and it will rise to 20BHN depending on the amount of arsenic in them.
No, lead is not steel and wheel weights are not just lead. Tin doesn't soften, it hardens. If you read Elmer Keith at all you will see a "hard" alloy for him was 10:1, lead to tin while a normal alloy was 16:1.
If you want to know more about cast bullets, there is a forum dedicated to them. The people there are great and will help you through just about any problem you have. Some will get kind of smart with you, but hey, you are the one asking the questions!
I haven't cast as long as some. I have cast long enough to know first hand how things work. Water dropping or even oven heating after casting and then water dropping will make bullets harder than you will ever need for most handgun loads.
What causes the worst leading is size. If you have too small of a bullet, it doesn't matter how hard it is, it is going to lead. If you have too soft of a bullet and drive them too hard, it is going to lead. The only time I haven't had a problem with leading is when the size of my bullets are at least .001" over bore size. I can make them as hard as I want, sometimes I add Linotype (which has a high tin content), and shoot them slow or fast, and I get no leading.
I have some pistol caliber carbines I throw lead chunks out of that are traveling over 1800fps, no leading. Why? Because they fit the barrel, period.