Right, we DO go by what is shown. WE ARE SHOWN that the director say no need to make his arm appear as it should it the bullet catching feat. We clearly and without ambiguity know the didn't try and speed up his arm to make it what it would actually look like. If he did, Ozy's arm would be a total blur and we'd see nothing. Yet, he didn't do that, in that all the fight scenes he used real people.. human people to choreograph the fights, not CGI. So yes, we DO go by what we see. I'll ask again, since you avoided the question....
How do you KNOW he wasn't fighting as fast as the bullet catching scene since we can see his arm move in both?