it seems to me that at high IQ levels the "necessary-for-A+" IQ tops out and conscientiousness starts to dominate, while it's the reverse with low IQ -- if you're just not smart enough, conscientiousness won't help much.

I low-confidence-agree with you. That seems gut-level correct, and fig 1 supports this notion but I don't know the extent that trend should be trusted and there are other possibilities.

If I had to guess I'd rephrase it as " both have diminishing returns, but the diminishing returns on IQ are both more dramatic than those on conscientiousness."

actually, looking at figure 1 it doesn't look like self discipline has diminishing returns at all, whereas it looks like IQ does. Just eyeballing, the "IQ test ceiling" for GPA is the 3rd quintile of this population's OLSAT7 lv G scorers whereas Self Discipline doesn't even hit a ceiling. But conclusions via eyeballing would be criminally non-rigorous :)