My concern in changing bolt grades is the fact that BMW has put millions of miles and $s into developing the E53 chassis. Those bolts attach suspension components and the stiffener plate to the front chassis horns. During development/crash testing/DOT-TUV certification BMW may have seen the need for fasteners that shear/snap when stressed during a frontal impact or contact with substancial road debris to protect passengers and control deformation flow away from critical components and central occupant structure. I don't know this for sure, but would be willing to bet the farm that there are structural engineers at BMW that do. BMW doesn't use a Magic 8 Ball loaded with bolt grades when they engineer/develop a vehicle.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!