TopGun,
The clutch between the sun gear and the motor/generator I mentioned is to keep the rotor from disintegrating or freezing and it has nothing to do with efficiency. Anyway, I do not think Toyota will ever need to put a clutch there. The efficiency the MT article mentioned, I think, is more to do with electric side of the system, i.e. higher rpm means higher drive frequency and higher frequency means higher hysteresis loss and eddy current loss rather than mechanical side such as bearing loss and wind resistance.
Putting aside the question of efficiency of the serial drive system, I think there is not much difference in efficiency between the Volt system and the Prius system once ICE and wheels are rigidly connected with gears.
I have been on a Volt forum in the past almost three years and one of the first to find the GM patent application. Many members, myself included, were disappointed to find the Volt used a Prius-like planetary gear setup, but understood the reason why GM had to use mechanical linkage between ICE and wheels to improve efficiency.
I hope your headache is gone now. Or, is planetary gear system too difficult for you to understand?