This is so weird. So I bought a used 2018 ZL1 last month. It came with the exact same tires only he replaced the front tires as well. So, after I got the car aligned a ton of the sway disappeared. However, after feeling the car out a little more and having a little more drive time post-alignment I feel like it didn't completely eliminate it.
WOT, as you say, right around 75+ MPH the car just feels like its floating at times. I don't feel it otherwise, and it doesn't matter which performance mode I'm in. I've even rotated through the various traction control modes with no luck.
I posted a thread on here about the car and the feel, feel like it pretty much just needed to be aligned with the general consensus but I totally get what you are saying and I have the EXACT same tires on the car. I don't think its a coincidence.
Here's the link to my thread:
https://www.camaro6.com/forums/showthread.php?t=577693