From your issue description, I can't see how it could be the rotors - were you having any trouble with them before? If not, a tire change would not likely cause a problem with the rotors, unless the lug nuts were seriously mis-torqued in remounting the wheels or something else happened in the service process.
Have you tried looking in the wheel wells for any obvious rubbing marks? On the tires? You say the tires are not "Porsche Approved", but are they the correct size? If they were oversized then you could end up with a rubbing problem, but lack of "Porsche approval" should not in itself cause a problem (Porsche can't test *all* tires on each model).
If you have any more details on the problem, someone might be able to provide better guidance. It is funny that the issue would not repeat when you were at the tire shop...
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