My motivation for asking that question is that, since extreme biological cognitive enhancement could reduce existential risk and other problems by creating people smart enough to be able to solve them

Smarter people can also come up with more dangerous ideas so it's not clear that existential risk get's lowered.

How efficiently can money and research be turned into faster development of biological cognitive enhancement techniques such as iterated embryo selection?

I think there already plenty of money invested into that field. Agriculture wants to be able to effectively clone animals and insert new genes. Various scientists also want to be able to change genes of organisms effectively without having to wait years till your mouse get's children.

Getting information information about what genes do largely depends on cheap sequencing and there a lot of money invested into getting more efficient gene sequencing.