Depending on whether or not the reader agrees that the stock 4.8is rotor is larger than necessary, this is looking closer to truth for this application. Not only is the M3 comp rotor smaller, it is also not as thick.

Stock 4.8is rotors are 356mmx36mm.
M3 Comp rotors are 325mmx28mm.

Now, for even more consideration, look at the 04-06 front E53 4.4:
332mmx30mm.

Still larger. The M3 braking system was designed for a ~3500lb vehicle. The X5 4.8is was ~5,000. I think this may be a bad idea.

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But size isn't everything. Let's consider balance. White papers on the concepts below are available here.

The reasons for my StopTech choice were clear enough, but why the four wheel kit not fronts only? Balance. We can't think of braking without considering what's going on at the rear. ABS systems are designed to apply measured amounts of pressure in the system based on what it 'knows' (was programmed) about the capacity of the system including the stock calipers front and rear.

Of course, the system will adjust. That's what it does. But it could be .5 sec behind while listening' for feedback from the sensors. As it adjusts, pressure is distributed across the system the way it was 'taught'. The system works, but the ABS system does not 'learn' about the new (in)balance of the system. Of course, what you end up with is this awesome 'feel' of more bite. This is bc the system doesn't know that the new, larger fronts have so much more brake TQ capacity and will continue to throw the same old pressure up as it always has to the OE front brakes. The result can be this awesome GRAB! and maybe even a little nose dive as those big binders squeeze with way more pressure than is needed up front- and not enough at the rear. Feels awesome. Doesn't work as well.

But then the rears start squawking. "Hey! Unexpected input! Wheels not doing what we expect! Need! Want! Hey! What ab us?", and ABS responds. ABS effectively evens out the braking pressure until the driver is out of the brakes. This will happen every time.

Of course, this is not GUARANTEED to happen in every front only kit, but it is certainly more possible when the braking equation is changed radically from front to rear. As above, it'll FEEL great. But will it actually be a more efficient system? When you start replacing parts on specs alone without considering the engineering equation you're changing, the chances are that you'll end up with a system that is not properly balanced- and therefore won't work as well as it should- perhaps not even as well as the stock config.

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