[hardparker]

Those Russian M5 adapters are pretty trick and would work just fine. I'll mention that all of of the cross referencing I've done seems to indicate that the last two generations of X5 and X6 brakes are direct swap bolt on with X3 an X4, at least in the front.

That would appear to include the X5/6M brakes. They range in sizes up to 395x36mm, and the calipers don't require adapters. They appear to be cheaper than M5 calipers on the used market, too. Other than mounting style and color, they appear similar to the M5 calipers for the six piston versions. The four piston versions are quite nice, too.

The rears on their site seem to use a blend of OEM BMW caliper and carrier from a 5-series and a custom bracket to fit the large M5 rotor. Pretty nice, and it looks like the only rear upgrade I've yet seen, as OEM parts don't seem to interchange with the X3 and X4 for the rear brakes as easily as the front. Thus the need for a custom bracket.

As far as 19" wheels, it's likely that many won't fit. Other BMWs running brake sizes between 385 and 400 mm seem to need 20" wheels, unless they get lucky with fitment.