Free-Air Concentration Enrichment studies such as soyFACE artificially raise CO2 (among other variables) and monitor plant response. SoyFACE, as the name implies, is focused on soy, an important food crop. Imagine a crop field surrounded by CO2 sprayers and heaters to simulate elevated CO2 and its effects.
Findings from the experiment include that increased temperatures will likely reduce yields of soy, even at elevated CO2. Higher average temperatures also increased susceptibility to herbivory by the Japanese beetle.
A related meta including 228 experimental observations found that barley, rice, wheat, soybean, and potato all have lower protein content at elevated CO2.doi: 10.1111/j.1365-2486.2007.01511.x
14 years of publications can be found here: http://www.igb.illinois.edu/so...
In short: even if water use efficiency were to increase, that does not result in increased yield, or crop quality.