Karlin tweeted - Are there any good articles making the case that IQ augmentation thru GWAS/CRISPR will be *harder* than Hsu/@razibkhan/@charlesmurray think?
That discussion thread mentions Kevin Mitchell's piece on mutation load (genetic load) as decreasing human intelligence from some ideal (Platonic form?) That is the first idea that came to my mind and it almost certainly plays some large role. But I'd like to suggest another way of looking at limits to IQ augmentation.
Allometry scaling is a way of getting at biological features that are constant across species by adjusting (typically) for body size. (Allometry seems to have some of the appeal in biology that physical constants do in physics.)
Sizing Up Allometric Scaling Theory (2008) http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1000171
Wikipedia for general background, https://en.wikipedia.org/wiki/Allometry
Wikipedia mentions (quoting here)
West, Brown, and Enquist in 1997 derived a hydrodynamic theory to explain the universal fact that metabolic rate scales as the ¾ power with body weight. They also showed why lifespan scales as the +¼ power and heart rate as the -¼ power. Blood flow (+¾) and resistance (-¾) scale in the same way, leading to blood pressure being constant across species.
The limit on total lifespan (i.e. maximum lifespan) of species being scaled to body weight suggests a limit across species on the evolutionary design of heart muscle (or cardiovascular systems.)
(Which by the way is why I find hummingbirds interesting, for if any species has broken the design barrier on hearts it is most likely them.)
It doesn't seem implausible to me that in the evolutionary design process of human brains those that are functioning at around 160 are already at or near the limit of maximum biological design, in the same way as heart muscle tissue -- Though with the difference being that humans are the solitary species that have reached such a hypothetical limit first. (I tend toward skepticism about the ability of current IQ test designs to meaningfully discriminate between IQ's in the highest ranges.)