Yes there is a noticable difference.

Your mileage doesn't change due to the installation as long as you drive the same as previously. If you take advantage of the new found power, your mileage will drop.

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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