[motex]

I do own a 3.0diesel and the first 2 seconds are slow, then when the variable geometry turbo starts to work its way into power, the best way to get into speed fas is to push the pedal to the metal.
When you push it to the end, you will feel you pushed a button, that "button" is called "kick-down-button" , what it makes is to send an agresive maping to the computer and gets the most power from the engine inmediately.
As for changing the gears, Im a former owner of an M3 qith sequential gearbox, the X6 is a superb gearbox and incredibly close to a sequential but in an automatic manner. Is a matter of getting use to it, first you have to switch to Sport mode and then start using the up and down paddles or the gear shifter, remember the RPM's diesel engines don't like to much RPM's, so try to change gears on around 4000 RPM's, you will feel the difference.
Another good advice is to get to know the torque peak, I usually change gears between 3800-4000 RPM's, that where the peak torque of my car is, after that, your are wasting momentum and not gaining any extra power or speed.
Well , hope this info will help you in some way