We had a ZX2 do about the same thing. Root cause was there was no check valve in the line to the booster (Mazda uses a valve that you can't see, inside a rubber line). Things were ok on the street, lots of part throttle and hence vaccum, so brakes felt ok. Got to the track and at the end of the front straight at Grattan, the pedal felt like a rock, no vaccum left after a few seconds of WOT. Ok driving around in the pits, ok in other corners. Put the right hose back in, with the check valve in it, (custom intake manifold, custom booster hose, opps.) and it was fine.

In your case, I think the check valve is the more traditional type, right on the booster, but I'm not sure. Worth a check though. It was confusing to us until we realized what happened.

Tim.