this thread is pretty interesting.
I would tend to agree with the notion that the rohloff is "draggy" and am not surprised that the top speeds are lower. I am a bit surprised they are so much lower.
Rohloffs are heavier, and are much harder work when riding in a club setting where wee accelerations are required often.
I ride a rohloff tandem (slowly) with my kids. Changing is dead easy for me, but we dont have fancy stoker freewheels to complicate things. My "power" output (such as it is) is much greater than my kids so if I ease off to change, then we ease off. Easing off to change is pretty important.
My rohloff solo is the bike I ride most, I enjoy it much more than my other bikes, but its a lot heavier and a bit slower. I think it feels slower than it is for many reasons.
Bringing back to topic, No reason I can think of to say why a rohloff is less efficient on a tandem than a solo.
Gear changing is a different matter: Longer cable runs, and co-ordinating easing off makes shifting harder, but then again derailler shifting on a tandem (front esp.) is harder than solo equivalent.