I don't shoot Canon, but from what I can tell the 5DIII uses post-ADC digital gain only after ISO 12800, so no point raising ISO beyond that unless you need 'properly' bright images SOOC. Looking at Bill Claff's graph, it seems that you do get a benefit in shadow SNR by raising ISO in-camera from 100 all the way up to 12800, although the law of diminishing returns needs to be considered after about ISO 2500: raising it from 2500 to 12800 only nets you a 0.2 stop improvement in shadow SNR at the expense of 2.3 stops of reduced DR: a decision best evaluated each time based on the situation (blips in the curve above 12800 are suspicious, as the data up there is often 'cooked').
The other take-away from the curve is that if you are after best SNR/DR it appears that you are better off increasing ISO one stop at a time (as opposed to in 1/3 or 1/2 stop increments) in correspondence of the steps in the graph: IOW if you are sitting at ISO 300 and you feel the urge to increase ISO a little, there is no point going to 400 or 500. Just leave it there with the benefit of slightly better DR until you feel the need to go to 600.
PS the 5DIII appears to have an effective QE of about 14% and PRNU of about 0.4%, plus a fairly noisy analog amplifier contributing about 8 ADU random noise throughout the range, so the last three bits never contain much information.
Your graph is interesting and can serve as a topic for discussion. It fits Emil's simplified read noise formula
R2 = (G R0)2 + R2, where R is the total read noise, G is the ISO, R1 is noise upstream of the ISO amplifier (presumably mainly the sensor itself) and R2 is the noise downstream of the amplifier (presumably with the ADC representing a major component). Roger Clark in his sensor performance analysis
(Fig 8a) implicates the ADC as a main contributor of read noise for the Canon camera. The amp noise itself is a tweener in Emil's analysis.
If I interpret it correctly, your graph shows a constant sensor read noise component which is multiplied by the ISO amplifier. What you term the amp read noise could include the ADC noise which is constant and adds in quadrature to the sensor noise to give the up-sloping total read noise curve. Bill Claff
states that a curved plot of the read noise expressed in ADU numbers indicates that the ADC contributes significantly to the read noise whereas a straight line plot indicates that the sensor is the main source of the read noise.
Bill's plots for the Nikon D800e are combined and shown below for ISO up to 1600. The read noise in electrons is relatively constant but the values are affected by the ADC readout and the noise in ADUs is linear indicating the source lies mainly in the sensor. The curves cross at approximately the unity gain of the sensor (ca 320), but I don't know if this is fortuitous or significant.
What do you think?