rx7 fc brakes onto AE86

[NoHachi]

40mins untill fade sounds like pretty damn good performance to me. And that with dubious brake fluid. After you cook the brakes, the brake fluid becomes a lot worse at its job. So ideally you should change it in between sessions (just a quick flush). Brake ducts (cooling vents directing air from the front air dam to the brake callipers) also can have significant effects.

I wouldn't be so fast about discarding the stock brakes, I would think about it if the rx7 kit had:
Larger then stock discs (don't care about the callipers, clamping force of the stockers is more then sufficient. (best you can expect is more even pad wear).
Lower total unsprung weight (forget about that one is you are running heavy ass 17s)
Cheaper and/or better pad-choices.