Additionally -- how big is the room in which you want to mount your TV? The loss coefficient for ultrasound in air varies with frequency, temperature and humidity, but it's about 1dB/meter in this range, give or take. If you have a three meter wide room, and you have the transmitter and receiver mounted on opposite walls as you recommend, then you lose 3dB = HALF of the power through the air, and there's nothing you can do about it. To get to your 33% total efficiency number, all of the other losses, including conversion from DC to ultrasound and back and transducer losses at both ends, would have to amount to another 15% at most -- highly unlikely, though maybe not impossible -- though these are all well known numbers that someone skilled in the field could write down for you.

There's a big difference between 30% efficient now and 90% efficient in the future, and "it's never going to get any better than this"