I solved the panic stop issue.
Seems like my pads and rotors were glazed (I had less than 1 yo Stoptech drilled and slotted rotors and ceramic pads, which had been recommended to me. No clue on how the glazing happened. A new set of OEM pads and rotors gave me some good bite back on panic stops.

The other remaining issue I'm still grappling with is a hard pedal on cold starts, particularly when at high elevation. Booster and check valve are new OEM. System has been flushed. Calipers are working. Rear brakes are adjusted. ABS is working.

The only thing I'm seeing now is that my engine vacuum at the plenum nipple is only 11 in, at a cold morning at 10,000ft. This might not be enough to clear the booster of any air. I started a new thread about that ...

- olaf