Forgot to keep you posted,
I bought a brand new GT86, had no problems with the stock brakes as far as fading/vibrations, but they really lack bite (require excessive pedal effort), and have excessive pedal travel.
I then bought DS1.11 pads for the front and the back of the GT86 and was a very happy man for a short time... Until they started vibrating when getting hot (after about 5-10 minutes of fast downhills). After stopping the car, the discs had massive deposits, just like in the Clio. And they cleaned each time, with street driving and cold braking.
Swapped them for some cheap stoptech pads, no vibrations, no matter how hot they get.
Just to remind you, I had no problems in the same conditions with the stock clio 172 brakes and DS1.11, but had the exact same issue with the DS1.11 and 197 brembos on the 172 (which was solved by swapping pads, even on the same used discs).
For those who mentioned brake bedding, let me assure you I tried every possible variation you could and couldn't think of
The deposits from those pads go away quickly after some cold braking, you can see it on the discs, and new deposits cause vibrations each time.
I've reached for the seller of the pads (someone really reputable here) with the full details, he sent the details to Ferodo, and this is their answer:
I have spoken to Ferodo and they said the DS1.11 is an endurance race pad, what will happen if you run them at much lower temperatures (on the road) they will deposit pad material on the discs and then they will vibrate. They need to be run at sustained high temperature. You will be better with a DS2500 for road use and swap to the 1.11 for track use.
This makes absolutely no sense, as you can see the discs clean of deposits after some cold stops, and it's "general wisdom" everywhere that "race pads" clean deposits when their cold, and only transfer deposits when they're hot (which again, can be seen on the discs after a session).
For now I've given up on these pads as there seems to be no solution.
@npt mentioned slotted rotors might solve this, but there are none for the GT86 where I live. Maybe
@LiamPa can try it and let us know?