My guess is they will be close to the same HP on paper with the ZL1 having slightly more real world.
The Corvette has bigger rear tires. 335 vs 305. The non Z07 tire is the Pilot SS and are very sticky. I had a Gen 5 ZL1 with around 600 hp and the Goodyears NEVER stuck like the MPSS do on my Z06.
Finally it will be power to weight. The Stingray weighs 3300 lbs. The Z06 weighs 3550 lbs. So the LT4 and other upgrades are 250 lbs on the car. This is after all the carbon fiber bits to make it lighter. This makes the car 5.36 lbs per hp.
The SS Camaro weighs 3750. Add the same 250 lbs on the car you get 4000 lbs. Which is my guess on the new ZL1 wieght. Even with 670 hp you get 5.97 lbs per hp. With 650 it is 6.15 lbs per hp. To equal the Z06 power to weigh it would need 746 hp which will not happen.
Also the 2016 Camaro is currently 54% front and 46% rear which I am guessing will be about the same in the ZL1. (Most ZL1 weigh add is in the front but betting GM will be moving some to the back to balance out like they did last time.)
Corvette Z06 is a perfect 50/50 so more rear weight on the tires.
Current record for a 100% stock non Z07 Z06 is 10.4 so the MPSS really put the power down.
BTW the car was in Aero 3 setup in the rear so higher drag. Bet the MPH could have been higher with the Aero 1 setup. But I would not be surprised if the ZL1 had a ton of drag too. The Gen 5 cars did. Downforce = drag but also handling.
My bet is the ZL1 will be a solid 11 sec car all stock with well driven auto examples pushing the 10s.