Brain function, colour perception, reaction times, rate of memory formation and personality are surely affected by slight sequence variations in genes encoding neurotransmitter receptors and biosynthetic enzymes. I'm suspicious of the language of limits. The gene pool has not changed significantly over the past 5,000 years but human society and individual opportunity have been transformed. History is littered with supposedly scientific pronouncements about the limits of women, African-Americans, southern Europeans, Asians, Jews. Science has done a singularly poor job when it has sought to define limits on human potential.
Humans are a remarkably homogeneous species.
By comparison, two random chimpanzees in Africa show a fourfold greater difference. Still, the residual 0.1 per cent between humans translates to roughly 3 million differences, with some conferring dramatically higher risk of Alzheimer's disease or breast cancer. Clearly, some DNA changes are unquestionably negative mutations, destroying the function of a crucial protein. But many of the common ones are likely to confer mixed blessings - perhaps an increased risk of infection by one pathogen, but greater resistance to another. Given the subtleties of human physiology, quick genetic fixes are likely to do more harm than good.