I read voltage regulators are also about 75% efficient so there isn't going to be much difference between using a regulator or a dc/dc converter
I wonder if it is a good idea to have two regulators.
One that will convert the parallel pack from 3.3V/4.5V down to 2.6V. I will run the LED's and Teensy from this. I thin Paul said the Teensy 3 can run down to 1.8V. I will see if 24 MHz is fast enough for me. I saw I can get LDO regulators that have a difference of 0.1V between the in and the out. I might be wrong. Since the conversion isn't huge maybe I won't have such huge losses
I can then have a second 5.5V regulator on the pack in series that might be 6.6V to 9V to power the sound, screen and the motor. I am currently using the 1.8'' LCD screen from Adafruit, but considering moving over to the eInk screen. I think I can power the eInk screen from the 2.6V regulator. I won't have the cool graphics that I was planning, but the kids don't care much about that, they just want to see numbers and I want to save battery power. The 1.8'' screen is a bit difficult to read in the sun, but I knew that when I went this route. So if I power the screen also from the 2.6V regulator then I will only have the sound and motor powered from the 5.5V regulator and since the sound and motor aren't constantly on I am not losing that much energy