I have not done any research on this topic, so I can't from my own experience. I know that there are many researchers who are studying this and they have shown differences in RUE among Hybrids, but I can't cite individual studies (although I am guessing there are a lot of them).

In the rice study we conducted, we sampled both above and below ground biomass. This was reasonably easy because the root mass was concentrated near the surface of the soil. Other crops would be much different. If you had to sample below ground biomass of winter wheat, for example, we found considerable water uptake at 1.5 m in a study we did a few years ago, and these roots would be impossible to sample. For this reason, many studies only correlate above ground biomass to light interception. This would not allow a one-to-one correlation as I showed using accumulated CO2 converted to carbon uptake vs. actual biomass accumulation, but it would provide an adequate RUE estimate. I would consider using only above ground biomass in the case of peanuts and potatoes.