I noticed one was added with Shop Spezial but never thought anything of it, thought it might've just been a test car or something, but I've just checked and it has far superior performance to the standard Mustang but cannot be upgraded.
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The car with the TP Temp350 name is the bonus 'Stang
I've driven it and while it is obviously quicker than the stock Mustang, it's still far slower than the standard Mustang if it was fully upgraded. Although it can still have all the visual upgrades that the standard Mustang can have.