I think its optimization can vary quite a lot.
Some examples:
Telecom's biblet 2 has a Pearl, but there's a noticeable difference between that and the one of Kindle 4:
At the same time, the new T1 has a slightly more intense black than K4:
I think this is related to the way the software (waveforms) can manage the electronic ink particles.
Every PRS 650 / 350 vs T1 video comparison I've found on YT, shows a slighly washed text on the first, compared to the second
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I know you dont have a 9,7" that's why I suggested to make comparisons 6" vs 9,7" using epub and not pdf