Subie -- thanks for the answer, that's some good diagnostics.
Details of "fade": The pedal feels soft, but if I push harder, I can still get stopping power. As the session continues, the fade gets worse, in that I have to push harder to get the same kind of stop. If I pump the pedal, it gets a little better with each successive pump of the pedal. The symptoms typically disappear within an hour or two of leaving the track. Saturday night, after fading pretty hard in the 4th session, symptoms persisted until the next morning (more than an hour of freeway driving later and a night outside in 50F (10C) ambient).
If I'm boiling the fluid, then I need to keep my brakes cooler, so improve my ducting? And switch to a different brake fluid? (Amsoil wet/dry boiling numbers are 410F/580F, which seems like it should be more than adequate, and within a few degrees of ATE Super's points.) Do different brake pads transfer heat differently, or is switching pads likely to be a dead end?