Unnecessary expenditure in my opinion. My car has Mintex 1166 pads, excellent tyres (do more for braking than bigger discs under normal conditions). I have fitted Brembo slotted rears (to no significant performance increase) that are the standard size 300mm. OEM 302mm from rotors.
As tested by NZ Autocar 100kph-0 (62mph-0) in 30.92m (101feet). This was the 3rd fastest stopping ever tested by them at that time. Faster than a V10 M5, Ferrari F430 etc.
And at not a lot more than fitting standard components.
If you got them cheap and know someone that can fit the M5 gear, then you will get a better braking package. Pads & tyres are where bigger gains can be made.