Never as much as you would imagine. Always flush through about 600ml just to make sure. I recommend removing as much as you can from the reservoir with a syringe first as it will take a lot less time to flush.
I would not bother with racibg spec. Check the wet boiling points and there is not much difference between 5.1 and the race stuff.
I've used it a vehicle round Spa, and had no fade all day. I also use it in my Subaru and my Transit. Just make sure that the grade is suitable for the system and compatible with the old fluid that might be lingering.
The quality of Motul BF is top notch, and not really any dearer than the alternatives, concidering what it does