I have explained this before. When the diesel revs high on 1st gear there is no load, turbos dont spool and there is no boost. The boost actually appears sometimes in 2nd gear when the engine starts to load. X5's reaction to a sudden gas pedal stomp is even worse. When the required torque(gas pedal position) is less than the available torque (RPM and boost dependant) it unlocks the torque converter, making the engine to rev even higher It is even more annoying with a trailer, the engine revs on first gear, but there is no torque available, the car doesnt accelerate and the computer waits for the redline to upshift.
A real flash tune may help a bit, because there is more available torque at any given time and it makes the computer less prone to downshift. With a piggyback there is more torque available too, but the computer doesnt know this fact and acts like normal.