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I would check the weight of the wheels vs. the OEM X1 19" first.
While the wheel doesn't sit on the studs, they are still what's reacting the loads from the car. The 2mm difference in thread size results in ~ 27% reduction in the stud cross section area under loads from acceleration/deceleration/turning. It's most likely that BMW used larger studs to react the V12 7 Series' 553 ftlbs of torque and the + 5,000 curb weight or whatever the max torque and weight were for the 5 series. However, heavier wheels exert larger forces on the studs, throw in a few thousand cycles of acceleration, braking, and turning and you get cracks.
Not saying this will happen, just thinking why BMW would use two different size studs and how it affects what you're doing.
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