My X5 had a similar collision just a few months ago. First off using a non dealer recommended body shop will not void the warranty. I would recommend using a body shop who mainly deals with exotics though if possible. The one I used is basically does only German cars and exotics (Ferrari, Lamborghini, etc) Though I did not smell exhaust any more than before they replace basically everything in the rear end that was even dented slightly. Just to name the big parts: bumper, all of the mounting and structural hardware for the rear bumper, muffler, PDC sensors, and the heat shield. They also had to remove the rear quater panels for some reason so they also had to remove the headliner. The entire repair took over two weeks, and cost over $8000. Though all of it was paid by the other driver's insurance, as he rear ended the X5 in stop and go traffic.

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On Order:
2014 BMW X5 xDrive 35d- Space Gray-Canberra Beige Dakota Leather-Fineline Oak Wood Trim-ZDA/ZCW/417/4FF/4NB/5DL/688

Current:
2013 BMW 535i GT- Neptune Blue-Venetian Beige Nevada Leather-Fineline Matte Wood Trim-ZCW/ZVP/2ST/3AG/416/677
2005 BMW X5 30i- Kalahari Beige-Sand Beige Nevada Leather-Dark Burl Walnut Trim-ZCW/ZPP/ZRC/508/522/609

Former BMW's:
2010 BMW X5 xDrive 48i
2003 BMW 745Li
2001 BMW X5 30i
1995 BMW 525i