Thornburgh was an excellent Governor...
Dick Thornburgh is a good guy. He's not going to white-wash something because he had a dispute with Rove ten years ago. He's an old-guard Republican, dignified and moderately conservative. He won't have any agenda. He will simply report the facts and draw defensible conclusions.
In other words, he will slam the hell out of CBS.
Interestingly, I remember how he was the canary in the mine regarding Bush 41's loss to Clinton. He resigned as attorney general to run for the senate in Pennsylvania, and lost in a midterm election he had been heavily favored to win going in.