There are some pretty obvious ways we could improve human cognition if we had the ability to reliably edit or augment it. Better storage & recall. Lower distractibility. More working memory capacity. Hell, even extra hands for writing on more blackboards or putting up more conspiracy theory strings at a time!

I suppose it might be possible that, given the fundamental design and structure of the human brain, none of these things can be improved any further without catastrophic side effects—but since the only "designer" of its structure is evolution, I think that's extremely unlikely.