"A few years later I ditched the factory Dunlop front disc system and installed the ‘84-‘85 ‘Blake’ system on my car."
Good for you! But, I wouldn't want to encourage anyone to depend on the Dunlop discs to stop an Avanti under ANY downhill situation (with no booster). In my area I have to cross the Blue Ridge mountain on a road (Rte 33) that is steep, miles long, and with no place to pull off the road on 90% of those miles. Of course, one can downshift to keep the car speed down.
The original Dunlop disc brakes are safe if in excellent condition, but I believe in a safety margin, and that means modern disc brakes.
--Dwight