Lots of rumors have been popping up. Most of them are stating a source leaked that is was 600 HP or more. Jalopnik seems to be the original source which you do have to give them more credit than usual given that they brought the C7 to the world long before it was revealed.
Here is a link to a supposed spy shot of a Z06 testing...
Here is Fast Lane Daily mentioning it on their show and they are affiliated with Jalopnik. Given that the C6Z06 Carbon Edition was right on the heels of a Gen IV ACR with 505 HP, I'd be really curious how well it would do with another 90+ HP.
In all reality, we should view this as good news. This should push SRT to tweak the Gen V some more. Given that this will be a 2015 release, I bet the ACR will be as well. However, if the ZR1 gets a similar power increase, the Gen V all around needs to get to 700 HP, not just the ACR IMO. Anyways, this is exciting times as many have said. Looking forward to the ACR.