I have the NQSBBK fitted and run 18's, only because 18's came on the car when I bought it.
I have 225/40 tyres on them. I bought the 40 profile tyres to give me a bit of cushioning but then polybushed the car.
I have kept the standard front disc size rather than fit the larger Merc discs. As my car is only used on the road I thought i would save the extra weight of the Merc discs and I wouldn't be needing the extra cooling of the larger discs that I guess would be needed on a track.
I do plan on fitting cooling ducts though.
I see, that makes sense for your situation. How are the 18s working for you, are you going to to stick with them or do you plan on changing?
I am in a similar position, currently a road car but would take it on track in future with more knowledge and a better prepared car.
my car is bone stock and am going to be doing wheels tyres suspension and brakes before power mods, the NQSBBK would be my primary option for a caliper upgrade should I choose to stay 17”, 18” would allow me to fit the ml350 discs and cayenne 18z calipers.
Stock brakes for me are sufficient for 99% of road use so I’m not in a rush to change them but I’ve still gotten them very hot on multiple occasions and I don’t feel I can get the best out of my car with them. I wish our cars allowed left foot braking I miss using that.
Ultimately it boils down to how much I am willing to spend and as nice as a 2 piece brake disc setup is, £2000 is a bit much for me. I have more research to do on this though.