The problem is that the flywheel was warped, even though it was new. I suspected that after midlife mentioned it and it's the only cause that made sense. I had trouble with starting the car off the line, but after that it was fine.
First, I got an adjustable quadrant and firewall adjuster from UPR; what an amazing piece. I adjusted the clutch engagement and that helped a lot, but the problem was still there. On take off, it would bog like hell and shake the whole car, and on hard upshifts you'd feel it bog a little.
Removing the flywheel, having it resurfaced, and having Pete at Star Performance replace the clutch since it was on warranty (nice, knowledgable guy), the car ran fine.
I drove it a bit even though the problem was there. It was fine if you slipped it a little on take off.
Lessons: never assume anything is within tolerances, 5.0 Mustangs are fast as hell, 5 speeds are Godsends, old women don't like Flowmaster 40 Series mufflers, Jap cars are pathetically slow, even when modified, 300 ft-lb of torque in a 3100 lb car are more than enough on local streets (although I want to add more soon),
street racing depends more on the driver and not the car (but I knew that before I beat the two M5's I raced on separate occasions)...my aftermarket springs help a lot keeping the car flat. They're 750 lb/in with the rest of the car a stock suspension.