I'm well known to be a persistent b&stard when I want to find out why something is the way it is for a technical reason
It turns out that
The 520i doesn't need cooling for obvious reasons
Everything in between does
The m5 doesn't need ram air cooling because the disc has both rear and front (of disc) air passage. This lets the disc do it's work independent of ram air ducted (motorway) speed. makes sense for a car that can be just sent to twisty's.

But that doesn't seem to be the case in real life, standard M5 brakes were the weakest link in the package (start to fade early) but the big brake conversions handled that - all depends on what you do with your baby.