I think you need to go back to square one and ask yourself why do I have to have a 30 BHN bullet for XX pressure and XX velocity?
Prior to powder coating, have you noticed flyers with softer gas checked bullets with adequate lube? If so, then perhaps your bullets are slumping and you do need a higher BHN.
Are you experiencing leading with PC bullets at high pressures and velocities? If not and your bullets aren't slumping, why do you need high BHN bullets?
I don't even test the BHN of my bullets anymore but I can get 1-2" accuracy @100 yards out of my Lee C309-113-F bullets at 1800 fps. That's only 16 kspi peak but I regularly shoot the Lee 356-120-TC out of my 9mm guns at 35 kpsi and they work fine too.
Rather than try for "unreachable" BHN's, see if what you already have works?
I also read that it takes a couple of weeks to stabilize hardness. Did you test your 28 BHN bullets a couple months after casting and did you test your 12.5 BHN bullets long after powder coating? I also take it that you are cutting away at the powder coating to test the hardness of the actual lead and not testing the hardness of just the coating itself. Sorry if I missed it.