I was only commenting on the relevance of using an M3 engine in your E53 based on the fact that a race car used one.

I think there is a parallel comparison that could be made, between three generations of RAID off road racers, and their road-going equivalents (using the word equivalent rather loosely). The X5 is about ten years old. They were superceded by the X3 CC, which was lighter and faster. Did much better. The X3 CC was now been replaced by the Mini Countryman All Road. Again, lighter, faster, more competitive. While these are all race versions of street cars, which not much in common, it does suggest that the best way to be competitive is to start with a more competitive vehicle, ie lighter and faster, and not just bolt hp into a heavy platform.

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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