Here we go again 
It modifies the gas pedal, not the engine. The several-millisecond response of the drive-by-wire throttle can be reduced, and some posters note that they can feel the difference. No impact whatsoever on power, and no direct impact on efficiency. Intuitively, there could be a reduction in fuel economy due to more aggressive throttle application, as your best fuel economy will be with gradual throttle responses and this mod is designed to make them less gradual.
The only warranty it will void is the one on your gas pedal, or any consequential damage caused by the system failing and operating the throttle other than as you expected based on the pedal position.
The link posted above summarizes it well.
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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