Iirc the only time Katsuki has ever expressed any concern about keeping up with Deku’s OFA development was in regards to Deku getting faster with his higher percentages, not anything about the extra quirks, all of which, as you pointed out already, he can pretty much already replicate
and he really didn't seem that concerned about it at all when he was thought-monologuing about this. he ended his train of thought with a very firm "I ain't gonna be left behind."
two other things worth nothing here also. one, the flashback from chapter 284 puts this in a different context than it initially appeared to be. he's monologuing about keeping up with Deku not out of rivalry, but because he's worried about Deku's safety. he's preoccupied with keeping pace with him because he wants to protect him. (and of course, he doesn't breathe a word about any of this to Deku, and he's got his bullshit "I'M JUST HERE TO BEAT SHIGARAKI" excuse ready to go, because god forbid he ever let on that he actually cares about him, sob.)
and two, he literally gets his own speed upgrade like thirty minutes after this lol.
Deku's not the only one who's still getting stronger. Katsuki is also out here spontaneously activating new abilities which in his own words are "nothing like [he's] ever done." safe to say he definitely hasn't reached his limit yet either.
