I haven't seen anything from Floyd that would make me think KT could catch him with a big shot and knock him out though. I think anyone who says so is basing it on pure speculation.
Floyd's got a chin and a rolling defense to absorb shots well to minimize impact when he gets hit.
If we're talking about the Floyd who injured his shoulder in 2002, the injury might put him in a precarious position to get tagged. But if we're talking pre-injury or 2003, Mayweather had perfected the left shoulder roll by then and I don't see KT landing an effective straight right well enough against his defense.