Not as someone who is doing actual DDE flash, but as a fellow X5 D owner I will explain you how this "boxes" work. These are so called "Piggyback" systems which intercept some of the input sensor signals and output control signals (to the injectors,wastegate control valves, fuel pressure regulator, etc) and modify them. Nothing in the actual DDE program is changed, but by modifying the signals the DDE gets incorrect picture of what is happening with the engine. As an example,sending lower than the actual boost value to the DDE it is tricked to raise the boost. By sending lower fuel pressure value DDE is tricked to raise the FP and the end result is somewhat higher power. In other words, yes, this type of the boxes work, but they make the engine to operate in less than optimal conditions. Like a low level flash tune, piggyback mostly bumps the midrange torque,by a lot, which makes the car feels very powerfull in daily driving conditions. Contrary, a good flash tune will take care of many, many other engine parameters and increase the power by optimizing the engine performance. The factory tuning is very much on the safe side, so there is a lot to be gained only from there. Well tuned diesel will be faster overall than one with piggyback, let alone all other sides of the story.