Reminiscent of those guys we have all met in our work lives who have clearly been groomed by senior management... they come in, spend a year in the trenches, and then every year after that is a new promotion. The kind of jerk-offs that feel entitled by rank to lord over all the actual experts and professionals in their field. But I digress.
And no, I don't have a source for the 50% figure. It's anecdotal. When his little talking head pops up throughout the course of a game to "consult" on whether a call will be overturned or not, he's gotten the final call right about half the time and wrong about half the time.
I could certainly be wrong about the replay thing but it certainly seemed that McCarthy got an incredibly lucky break. The rule needs to go anyhow, but he got lucky.