AC Schnitzer made some upgrades to current M50d engine and look of BMW X5 (F15).
If you want most powerful (at this very moment) X5 F15 go for Schnitzer version.
How about most powerful diesel version of M50d engine? Well just imagine AC Schnitzer increased its power to 430 Hp from 381. In order to achieve this they remapped ECU and optimized exhaust system.