Being an avid boxing fan and an avid RJJ fan I can tell you without a doubt that AS would KO RJJ in a boxing match. Roy does not have the speed or chin that he had in his prime. This match would be a a freakshow match in my opinion and just shows that AS does not want a real challenge.
Now if you match RJJ in his prime (approx 2000) against AS in his prime it would be a complete washout in favor of RJJ. Unfortunately his Heavyweight experience and age caught up to him and he immediately lost his biggest advantage, his speed and twitch reactions.
Check out the new MMA Forum app for the Android