I had the BMW performance kit on the front of my old E39 M5 and it was excellent - far better braking during daily driving than the standard setup.
The part no for the kit is 34110444738. You get 2 x Brembo 6 pot callipers, set pads, brake dust plates and drilled and grooved disks. They are a performance brake retro fit kit for a 1 series and new 3 series.
To make the kit fit you need to fab a bracket for the callipers - I got mine from a guy in Belgium who has fitted a few of these kits but I don't think he does them anymore. The new disks need modifications too, but really simple. The M5 has 2 screws to attach the disk to the hub, and 3 centre pins. You have to drill the holes for the centre pins and the hole for the second screw to attach disk to hub. It can easily be done by pressing/attaching the OEM M5 disk to the performance disk, and drill through the M5 disk holes into the performance disk. Also you will need to trim the existing brake dust plate.
Everyone knows the weak point on these cars are the brakes. They are great for everyday driving and 2/3 emergency stops if needed. But if you have a blast on the twisty back roads or bring it out on track they turn to muck
The new disk size are actually a little smaller than the m5 disk and a little thinner. Stock ones are 345mm X 32mm. the performance ones are 338 x 26 mm. I think most people are turned off by the smaller rotors (than stock) of this brake kit but apparently improved braking performance is achieved by hastening the conversion of kinetic energy to heat (pad sweep area biggest factor) and improving heat dissipation rate (rotor size, material properties, geometric design). Rotor size is, of course, a big factor. With this kit, rotors are, as I have already said, slightly smaller but pad sweep area is vastly increased via the 6 pot callipers. Also the amount of metal used in stock VS performance disk is almost the same.
There are a few m5 owners on the continent who run this setup and have had very positive results on the Nurburgring.
This kit will probably cost around half of what the AP racing or Stoptech kits cost. But another major reason why I opted to go this way over the AP racing/Stoptech kit is that if you ever run into problems and need replacement parts they can be got easily from the BMW dealer. Replacement parts from AP or Stoptech are big money and you could be waiting a bit to receive the parts.