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It really doesn't make much sense from a manufacturer's standpoint to offer just a front caliper upgrade -- most of the time, at least. It's like offering an cool new iPod, but it works on the old Blackberry operating system. Not many companies will risk hard-earned performance gains and their reputations using the OE components. It's not a good day when you hear someone complaining loudly about their brakes when its got your name shouting from the face of the caliper -- and then finding out later they had a rotor problem with someone else's components. There is no good way to recover from that, so why do it?
Although this can be done successfully on certain rear brake applications, the front simply runs too hot with the right nut behind the wheel. The aluminum hat and floating or strap drive disc arrangement provides quite an advantage that is totally lost with just a caliper upgrade. The pin drive composite Ate rotors are an improvement over a standard 1-piece disc in terms of weight, but are no match for the performance of a proper 2-piece arrangement.
I've loved many BMW's since my first one - an E28 M5 (if only the E60 M5 was as much fun to drive that hard!). That car had 4-piston front calipers and had great pedal feel. Nearly all of the current Bimmers are still on sliders, which stop the car well under "normal" use, but sacrifice pedal feel at the limit without question. Even though the 135i gets the a 6-piston Brembo up front, that caliper has shown to be fine for the street and inadequate for serious track duty. However, it does feel quite a bit better than a 335i with the sliders.
It's generally considered that anyone serious enough to change to fixed, opposed-piston calipers could also be serious enough to push their car very hard. If so, a full upgrade is the way to go. Brake upgrades are not for the car -- they are for the driver. If you don't drive hard enough to need the upgrade, then you either don't buy them or do it just for the looks if you've got to have them.
Even BMW has a whole boatload of accountants, so the engineers don't win every battle. They have got to make compromises for comfort and reliability -- call it the "customer experience". They do not sell track cars down at your local dealership (well, OK, Lotus does, but I digress...). However, there are race shops all over that will prep a car for their serious customers, and that always means serious brake upgrades and generous cooling for almost all car makes.
(Note: If anyone reading this is a bling dude, ignore what I just wrote and go out and buy the biggest saucers and the brightest red calipers you can find to rock behind those 22's on your ///M -- or maybe your 525i with those cool ///M emblems you got off of eBay baby!)
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