http://www.astrosurf.com/buil/50d/test.htm
Scroll down a bit and you can see the QE for some older Canon cameras. Pretty low compared to CCD or modern Sony or Panasonic CMOS.
Bayer Matrix means less light hitting the sensor. Also that 4 pixels are used to make 1 colour pixel. Mono uses all pixels and nothing is inhibiting light from hitting the sensor.
I did see a long list of modern cameras and their QE. As I recall the highest were the latest Sony's like the A7riii and A7riv. QE is up around 65%.
So marketing claims of 84% for a one shot colour is misleading as that is the QE for a mono chip not a one shot colour which are always considerably less.
I agree 84% for a mono BSI sensor is realisitic. Its not for a one shot colour sensor.
Greg.