These vents may have been present on a GT500 mule we saw earlier, but black cladding was covering them up. The photo was taken of a similar mule, or possibly the same one, in a garage or lab. You can see a few more shots of the prototype on Mustang6G's forum, and we have spy video of a mule below. Previously spotted prototypes have been wearing Michelin Pilot Sport Cup tires, which are more aggressive than the GT350's Pilot Super Sports. And then there's that big wing out back. Mustang6G notes that this is not likely to be a refreshed GT350, since dealers have been told that car will not get any substantive updates when the face-lifted regular-strength Mustangs go on sale for 2018.
The current thinking is that the GT500 will debut for 2019 as a sort of end-of-life sendoff for the S550-generation Mustang. A supercharged V8 is expected to make in the neighborhood of 800 horsepower, which would best even the Dodge Challenger Hellcat and possibly the Dodge Challenger SRT Demon. There's also still a possibility that Ford will choose to twin-turbocharge the engine, giving its EcoBoost lineup a new halo. We're living in the golden age of horsepower.