They are aftermarket, they can do whatever they want. Just like companies that sell you a cold air intake that adds 20hp.
Ken says they pulled out early on the 414.68hp run
The 441.54hp pull is in that video too
I don't give two fucks about any Nurburging times.
But again, that doesn't matter at all, because the whole comparison was the production GTD vs the production ZR1. I don't care about the times of your grandma's Buick
3600lbs, 394hp, it is an AWD 4-door, but it's priced similarly to the Dark Horse, can be had with the same Trofeo R tires, torque-biasing rear diff so it can go sideways, unique 5-cylinder engine...
0-60 is 3.2 seconds, quarter mile goes by in 11.7 seconds at 118 mph, manages 30+ mpg on the...
Yup, wait for a Harbor Freight sale, get a low profile Pittsburgh jack
Don't get the tiny 1.5 ton aluminum one, at least not for your only jack.
Then pickup a set of jack stands.
And a US General tool cart, at least the 5 drawer model, or one on of the rolling cabinets.
And then a bunch of...
Practice, technique, etc.
You should be able to go 1.9x on the stock tires, if you're in the 2.1-2.2 (or even slower), you need more time launching the car before you should bother upgrading the tires.
https://www.motortrend.com/how-to/1712-running-11s-with-a-factory-stock-2018-mustang-gt
You're likely going to be at 190-something and the temps will only go up when you floor it
Thermostat alone might bring you down a couple degrees in general but won't really help the car when it's over 170. You need things like a radiator, vented hood, etc
Testing data
https://trackmustangsonline.com/threads/is-the-cylinder-head-cooling-snake-oil-first-data-may-indicate-so.21353/
Also there are the people that say the cooling passages are enlarged in the gen3/4 blocks eliminating the need for anything like this