Most of Floyd's knockouts came at a lighter weight and early in his career. He only has 2 KOs since 2005, in 2007 over Ricky Hatton and in 2011 over Victor Ortiz, and that one was after a head butt when Ortiz was apologizing, had his hands down and Floyd hit him with a left hook and a straight right.
I think Floyd wins easily but I think McGregor takes it the distance.