I had 18x9et30 ARC-8s with 235/40/18 MPSS on my non-x E92, also lowered on KW coilovers. Up front, I about 1mm of inner clearance and had visible tire marks on the suspension, however the tires didn't show any wear and I never noticed any rubbing. I recommend trying the wheels first and then buying a small spacer if you think it's needed.
I'm not sure if E92 x-drive has different outer clearance than non-x though, I've only owned RWD E92s.
Edit: found the pic of my inner clearance, I would embed it but the image is pretty big:
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