Honestly boxers and the like are only good for say 12 rounds if at the top of their game i know they are expending lotsa energy but for stamina and fitness id put money on pro cyclists would be way higher up the scale.
I mean boxers can pace themselves and look good (im proof of that)and look seriously fit yet cyclists cannot do that unless they are caning it as they have others who will pass them if they cannot keep the pace as there is no kidding in cycling.
Well looking at cyclist training in boxing could do it not the technique but the stamina after few week be fitter than a boxer other way around boxer wouldnt achieve the fitness to compete in cycling i know one is fighting and the other isnt.