Hi Guys,
To add a datapoint, we have used a Wilwood kit for a while, 12.2" rotors with Dynalite calipers, and they work fine with the correct pads. At least up to around 210-215hp. The .81" rotors are the limiting factor in my opinion, that and the small pad volume that is required to fit any of the available brake sets to a Fiesta. We would go through rotors almost as fast as pads. The rings are inexpensive, but the 50$ ones cracked even faster so we started using the directional vane ones that were around 110$ each, and they lasted around 2 sets of pads. The pads we found best are the Wilwood BP-30. They last really well, and dont really fade before fluid boils.... BP-10 are awful, even for street, mu is just too low. never tried 20...
We have now gone to the EBC kit, and it is much thicker rotor and much more pad and caliper volume. They are larger and slightly tighter fit than the Wilwood, but everything that worked for the Wilwood kit has worked for the EBC kit. 15x9 wheels, 15x8 wheels, same spacers, ducted backing plates....
Our build thread has some details, or feel free to PM with any specific questions.