bluelines:

The GT3 is built by Weissach. It will not be a tuned S with a wing from the parts bin. Look at the last road legal cars that came out of Weissach: GT2 RS and GT3 RS 4.0. 

I don't now how this little myth started, but I recently read it somewhere else as well, so someone appears to be spreading it around. 
The GT2 and GT3 models were built on the same assembly lines as all the other 911 variants in Zuffenhausen. 
The GT models just differ from the other 911 models in as much as the GT models were developed by the Motor Sport department in one part of the Weissach R&D facility, whilst the other 911 variants were developed by other teams also in Weissach.  


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fritz