When BMW pioneered the fastback SUV with its X6, it wasted little time bringing out an M version, figuring correctly that buying an X6 isn’t about making a rational choice—it’s about making a statement. And the X6 M makes that statement louder and prouder, a preening, muscular brodozer among SUVs. Unsurprisingly, BMW applies the same logic to the generally not-very-baller compact-luxury-crossover segment. It brought out the X4, basically an X3 suited up in a flashier—and considerably less practical—fastback body that looks like a junior X6. The next logically illogical step was . . .