Prime for prime i don't see it, mainly because of footwork. Prime Floyd gets on his bicycle like in the Castillo rematch and outscores Tszyu who was slow on his feet and pretty stiff in his upper-body. Tszyu would get in a big punch or two along the way but Mayweather's underrated toughness would see him through to a UD. Prime Tszyu against current Floyd may be a different story though.
Kostya Tszyu was too stiff and mechanical, had good timing and power, and too me was essentially a counterpuncher. I think Maywether's speed of foot and hands would have been too much for Tszyu. As for the whole ducking thing, as I remember it, Mayweather called out Tszyu before his fight with Hatton, then said he wanted to fight the winner of Hatton vs Tszyu. When Hatton won he and his handlers didn't want the Mayweather fight, which would have been at 140 at that time.
Action, not words. Floyd talks about fighting top guys, but then fights bums. Floyd was the one who could have made the fight happen with a snap of his fingers, since he was the American star. Kostya was a Russian kid who just took whatever they gave him. He was never a trash talker. Facts: Floyd could have made the fight. It never happened. To me that is what Floyd's career is. He never fought Paul Williams, Antonio Margarito, Kostya Tszyu, prime Miguel Cotto, Manny Pacquiao. He talks about being better than the best fighters of the old days, but he never even beat the best guys in his own era.:deal