It seems to me, viewed a certain way, the human body is almost entirely driven by emergent phenomenon. I have serious doubts about our ability to edit the source code to achieve very straight-forward single side-effect modifications.
We may very well be limited to merely adjusting "errant" DNA sequences that cause or are implicated in disease or undesired attributes. Who would choose a short child? Or one with myopia? Or one with weak muscles? Or even one with the misshapen chest and facial orifices necessary to survive lower partial pressures of oxygen?
The long term effect of custom genetic modification actually seems like it would just be used to reduce the overall genetic diversity of humans on the whole, not to improve it in drastic ways.
In fact, there are economic incentives to create broken clones that require daily medical attention so you can turn them into slaves that work for medicine.