Agree with @Fergy To me Mercer was a hot/cold on/off fighter. At Mercer's best, he wins. 2 out of 3 goes to Chris.
Mercer was so inconsistent, its hard to envision him beating a slippery cagey guy like Byrd who had so many superb performances against big guys like tua, Vitali, etc. Mercer actually struggled with and lost rounds to Bert Cooper and Damiani, but somehow he gave Lennox Lewis a very tough fight. Byrd always seemed to be in shape, focused, and was a true professional sticking to his game plan.