After publishing this project I got aware of the WS2815 LEDs, which can be powered from 12V directly. These LEDs have been released a year ago and I somehow missed that.
So I checked their datasheet (http://www.normandled.com/.../WS2815%20LED%20Datasheet.pdf) and I have an interesting observation.
Note that the following will be just a theoretical observation and since I don't have these LEDs, I cannot perform any real-world measurements.
I bet that Neopixel Booster + WS2812B has better efficiency compared to WS2815. It seems that there is an internal 15mA linear constant-current regulator which is then dimmed by PWM. For the blue channel with that means that 70 % of the input power you burn into heat and then 30 % of the power goes to the LEDs. Meanwhile, with the NeopixelBooster solution, you step-down the input voltage to 5 V with 90%+ efficiency, and then you have the internal 15mA linear constant-current regulator (70 % of the energy goes to LED) - thus roughly 60% of the input power goes to the LED and only 40 % is turned into heat.
The LED itself will have an efficiency of ~15-30 %, but it is the same for both cases, so I excluded it from my comparison.
I should get some WS2815 and verify my observation in practice.