It would have been an interesting and likely good fight from the perspective of two skilled and fast boxers competing. I don't see Khan beating Floyd, but I see the early rounds being interesting and a high speed, tactical match up with Amir gassing and Floyd taking over in the later rounds- potentially stopping him. Floyd stopped knocking people out when he rose two or three divisions above his best weight- at 140 and below Floyd was stopping elite guys more often than not. Can't really fault him for not stopping much larger, elite fighters like Oscar, Baldomir, Canelo...etc., especially considering all of those guys were durable and had great chins at the time of the fights.

Styles make fights and Floyd had a harder time with boxers with good speed- watch his fights vs Burton, early vs Judah, and early vs Oscar to see what I'm talking about. Not that he didn't or couldn't overcome those types of fighters, just that Amir would have presented style problems early for Floyd. Floyd would have likely attacked the body and fought at a pace that Khan couldn't match, exhausting him in the process and letting Floyd take over late. Khan had stamina issues and one thing Floyd never lacked was lungs/gas in the tank- the guy was a machine in training/fights and outside of maybe BHOP was the greatest conditioned boxer of his generation (BHOP may get the nod from me since he did it at such an advanced age).