I have run both NGK TR6 and TR7IX gapped at .028 for 16-17psi on 93 octane with no additives. For a well-tuned street car frequently run to its full potential, I have found that replacing spark plugs every 2 years or 10,000 miles (whichever comes first) works great for me. I have never experienced poor drivability due to a worn or fouled plug. My engine initially runs smoother after a cold start with the TR6s, but other than that, the difference between the two plugs is undetectable by me. As Chancey brought up, routine inspection of the plugs on a modified Shelby is important, looking for signs of detonation and oil burn in the combustion chamber, to detect an impending catastrophic failure.
My TR6s start to show wear at 10k miles:
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My TR7IXs looked good at 10k, could have gone 20k:
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Looks to be expected at the milage