You say the Roy that fought Griffin, but you also have to ask yourself which fight. Remember he immediately rematched Griffin and DOMINATED him, so to think Roy changed with the passage of time, at least up until that point, is a falsehood. Roy is like Calzaghe in that he had the tendency to fight down to another fighter's level, or get bogged down with outside distractions, but when he was focused he was impossible to deal with.

On the other hand, Floyd has proven that it doesn't matter if he's the slower fighter. He has the maturity of a fighter several years older than him and can maintain his composure even if he's losing. One of two things would happen:

Roy jumps on Floyd in the early rounds, picking apart his defense with his superior speed and slipping what little activity Floyd gives back until he catches him with a shot that leaves Floyd hurt and out after Roy flurries him after the 1st knockdown.

Or, Floyd realizes the first rounds will be Roy's and gives him just enough pressure to make Roy burn himself down to a pace he can deal with by round 7 or 8. Then Floyd uses his accurate, one at a time punching to slice Roy up until he evens the fight out by the end and hopefully gets a SD.

Or, Roy, taking a rest in the middle rounds, sees Floyd coming on and rallies in rounds 10-12 and comes on strong to batter Floyd back, takes at least 1 of the 3 rounds and wins by SD. Prime for prime, I think Roy takes it on sheer activity alone, but Floyd fights the fight of his life and takes 4 or 5 rounds.