I was able to start getting plans from level 1 hard at 7mill power.
Currently at 11-12mil power and I can kill the 3rd extra boss on level 2 easy for the ironheart heart plans (need 3 more), but at 55mil power I can't kill the 4th extra boss from the 800 score.

Herby I think level 1 hard 4th extra is around 35mil, but its been a while since i've done hard.