You can run a staggered setup on xDrive, but I'm telling you right now to not. There are a plethora of things that could go bad, and just to list off a few known after-affects
1. When your front tires are smaller than the rear they move faster and have less overall area. As a result, the AWD system will think your front tires are slipping and thus they will cut power.
2. Increased/Uneven tread wear because the car thinks that fronts are going faster, and then will cut power to the front
3. Performance losses
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