I finally found one source at www.oit.doe.gov. Interestingly enough, it stresses the importance of eliminating voltage imbalance, but cites very low efficiency gains.
It referred to tests on a 100 hp motor at 75% load with a 1% voltage unbalance. It predicted current unbalance in the range of...
comapro: I also want to quantify the economic benefit of balancing phase currents. I work on air conditioning systems in office buildings and often measure imbalanced currents of 10 to 15 percent. These are usually accompanied by voltage imbalances of 1 to 2 percent. My buildings are dominated...