From what I've read, you can eliminate the master cylinder, booster and pedal linkage as the problem. If you have a firm pedal and no leaks, then the master cylinder is fine. If you can drive normally and stop normally then the brake booster is working. If you are getting a full range of motion to bleed the brakes, then the linkage is also working. Sounds like your problem is only under hard braking. Do I have this right?
So what's left? Pads, calipers and rotors. Early in the thread you mentioned that you replaced the pads. How many miles are on the pads now? Did they get bedded in properly? You said that the rotors still have thickness but have some rotational grooves in them. If the rotors are dished or grooved the pads will not seat correctly. The rotors will develop hardened groves and the pads will never feel right.
I learned this lesson first hand. After rebuilding my calipers I replaced the pads using rotors that were slightly grooved. There was an improvement in the braking with the pads but I soon had erratic braking once again. I finally decided to replace the pads and rotors. That way everything was straight and true. I also made sure the caliper pistons were aligned at the proper angle.
Viola, I had the awesome braking power that Porsches are known for. After a session at Drivers Education event, I was practically giddy since I could break hard before a turn without locking up one wheel. I could also brake later because the pads were finally bitting.
I've had two carreras and both requried more pedal pressure than our '91. It can be quite a workout if your used to newer cars. I have to use twice the force to stop my '87 Carrera than with my '00 Jetta. Nothing will stop the Tahoe because it's a POS. I know because I have one.
Good luck!