Stay away from Cross drilled. Read the attached file.
Your BMW calipers are very good. Upgrade the pad compound, fluid and get better tyres. Get braided brake lines fitted. Make sure your brake ducts are functional (if fitted).
I have got Brembo rear vented and slotted discs. I honestly didn't notice much difference. I just liked the way they looked really.
As I've stated before my system is fairly standard, Mintex 1166 Sprint pads, ELF fluid, and Brembo slotted rotors and tyres. 100-0kph stopping time. 30.80m! Ferrari F360 34.59m.
The 540iS weighs more than the Ferrari also. A light car will not generate as much heat in the discs as a heavy car, so heak soak should not be so significant for you.