heat treated doesn't harden all the way to center
From all of the metals industry reports I've read that's wrong, steel when heat treated is a surface hardening. Lead reacts the opposite and when HT it is the same hardness all the way through. The only exception that I'm aware of is surface work softening such as sizing the bands.
As for expansion it would depend entirely on your HT technique. If you heat in the oven to just below the slump point and quench you will acheive near max hardening, about 30 BHN. At this the boolit would in all probablity break apart. Water quenching from the mould should give you around 18 BHN. Your velocity will play a key role.
This is with WW and I've never tested the results using Pure and WW. Reducing the antimony percentage by diluting 50/50 Pb/WW WILL increase the hardening/time curve. Testing I've done with WW and reduced antimony indicates that final hardness wasn't changed with HT but the time to harden was increased a lot, from 3-4 days to 3+ weeks.
Here's an article that describes this.
Cast Bullet Alloy
Rick