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I believe they are 10/32 new (fronts) and 9/32 (rears) and should start pondering replacing by 3/32 or so depending on your driving habits (drive in frequent rain, etc) ... so yours are closer to gone than new. They are certainly still safe, so replacing is not currently needed, but is up to you if replacing them now adds peace of mind. Also keep in mind, 911's as a general rule will go through rears twice as fast as fronts so provided you don't always replace all 4, you'll be in this spot again at some point.
You might be able to talk the dealer into covering it if you point out this phrase in the Porsche CPO checklist (screenshot excerpt of the real checklist that they should go over with you). I doubt all four of your tires are now the same age as required here. You could ask to speak with Porsche corporate to double-check their approach if they don't agree and see if that moves them along. $550 is $550.
I like having matched tires too so I understand your dilemma. If they won't cover it and you decide not to I doubt you will have any problems unless you don't drive much and the fronts end up aging out (get hard) before the rears. I bet you can win this argument if you sounds like it is the only thing keeping you from being 100% satisfied. You can always say you checked on the forums and have learned that the fronts should have been replaced too. Dealers get a little scared of forum chatter about them.
I have my original P-Zeroes on. at >15000 miles. The rears are 4-6/32. The fronts are > than 7/32. I use 3/32 for my judgement. I took a 1000 mile trip last week. I am not sure MPS4S are correct for a Carrera GTS driven on the road. Nvertheless, I respect Dr. Cable's insight.
If you are just going to drive around on the street put some new meats on it. If you're going AX it and tack it to the track go burn down the current rubber some more before you replace it.
Newest Michelin offering. Suppose to be an excellent tire.....surpassing the Pilot Super Sports.
+1 Super Sports were the best street tire out there, the 4S is simply an improvement (wet) on that ... distinct from the MPSC2's which are primarily
a track tire...
or maybe try the new Corsa's they optioned on the 991.2's that knocked several seconds off of its Ring lap time!