Cool. That's one and that's good. Thanks for posting it. I've been looking and not found others, so yours is evidently the quickest while being the only one known to run 13.2's stock. It's amazing that it's gone 13.1 to boot!
I don't expect the same to happen with a factory stock '05 MustangGT at all. Then again, I didn't expect it from a GTO. Still, if someone gets a new GT to do even 13.4's I'll be impressed and then some.
The drawback to the GTO's with regard to 1/4 mile times is most haven't run better than 13.5's as of yet in factory stock setup. All that means is many MustangGT's will be a close race, left to the drivers to decide. I expect both cars to average about 13.6's while stock. The Mustang's ECM seems to "conveniently" limit it's 1/4 mile performance and that means modification for anyone hoping for great ET's. The Corvette seems to have the same issue and sadly, the next GTO will likely face the same scenario. That's GREAT for you though, as it will surely keep you in the running while all are stock.
Good luck with your GTO. Hopefully track conditions will hold out with the great air temps and you'll get that 1.8 short time again. That's OUTSTANDING, btw. Most seem to be stuck in the 2.1 range without better tires. Getting a 1.8 with the results you've seen will clearly net you 12's. Simple math tells us if you get a 2.00 on a 13.1 pass, you'd get a 12.7 on a 1.800 short time with everything else being equal. A 12.8 is definitely in the probability zone.