The aftermarket tuner closely associated with BMW offers the B4 Bi-Turbo (at least in Europe) with that same 3.0-liter twin-turbo inline-six massaged up to 410 horsepower that starts to pose a threat to the M4. What's more is that the Alpina's 442 pound-feet of torque eclipses the M4's 406 lb-ft, to say nothing of the 435i's 300.
Best of all, Alpina just confirmed via Facebook that it'll offer the B4 in convertible form, bringing the resulting BMW Alpina B4 Bi-Turbo Cabrio to the Geneva Motor Show. We'd expect the weight penalty to knock it slightly off the coupe's quoted 4.1-second 0-60 time, but not by much. If only BMW were to bring this one Stateside like it does with the B7 sedan.