I'm late to the convo, however, here's a rundown.
The Corvette C8 Z06 is not putting down 610whp, even after a cooldown period, unless it's in a controlled environment. It's closer to 590whp.
The Blackwing makes a little bit more power (around 605whp), and loads more torque (around 615lb-ft vs 419lb-ft) than the Z06.
Supercharged versus Naturally Aspirated. The C8 Z06 needs to get all the way to 8400rpm to achieve it's max power, while the Caddy needs just 6400rpm. The LT6 may Rev quicker than the LT4, but it doesn't quite make up for a 2000rpm disadvantage (I assume). Also, it helps that the Caddy gets max torque at 3600rpm while the vette's at 6300rpm (manufacturer claims)
(This one may surprise some people) the CT5 V Blackwing has less drag than the C8 Z06, no matter the configuration. The fact that the Z06 tested here was the one with the carbon fiber aero package, it made it quite a bit worse for the vette.
The Z06 tested here didn't have the Z07 Package, which means less sticky tires (for the launch), and more importantly, doesn't have the carbon fiber wheels, so it has a lot more rotational mass.
I hope this helps