I've had very favorable experience with Michelin over the years.
while the sizes are BMW-spec, the tires are not. being 'spec' has to do with how the tire's design and composition marries up with the vehicle's driving characteristics. the engineers spent lots of time on the track figuring this out, and since Porsches and BMWs aren't the same, their respective OEM-spec tires aren't made the same, but I doubt that'll hurt driving in the real world (as opposed to the track)
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