Power Disc Brakes not working on hard stop?

Thank you everyone! After reviewing all the responses and evaluating my experiences with 2 systems (SSBC conversion on a 67 RT Coronet (SSBC uses the same setup on B & A bodies) and my 67 Dart (Stock MOPAR setup), I believe I'm experiencing the natural limitations of the smaller rotor and disc brake system. Simply put, its at the end of its braking power. The only way to overcome it is to go to larger calipers and rotors, which would require larger, bolt pattern with larger wheels and tires. I've traded emails with both Dr Diff and SSBC and neither can offer any cures with my current setup. Steps I've taken: Power bled the system, measured the MC pushrod to ensure max fit and travel, checked all rubber lines, checked for vacuum leaks and found none, ensured booster and MC operate as expected. The brakes work great under normal circumstances with good stopping power and easy pedal. When I brake aggressively while going fast I reach a limit where more pressure doesn't provide more braking ( a hard pedal not on the floor). The fact that I have the luxury of testing 2 different cars with the smaller calipers and rotors and they act similarly leads me to believe this is normal. While I was hoping to find a way to improve I discovered this is the best 1960's technology had to offer. I appreciate everyone's contributions.