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The Efest 26650 purple 4200mah pushes a bit harder than the Shockli when fully charged but quickly drops below the Shockli for the rest of the discharge curve. The Efest is much weaker than the 40T at 20A. I think the reason why you observe the light to be brighter with the Efest 26650 is because the light doesn’t step down as fast since the Efest doesn’t heat up the light as quickly as with the 40T.
http://lygte-info.dk/review/batteries2012/Common26650comparator.php

Regarding Molicel, Mooch rated their P42A as the best all-around 21700 and the P26A as the best all-around 18650. Molicel is an excellent brand but they don’t make 26650, and I recommend not to use any 21700 cells until it is confirmed the longer length will not permanently crush the springs (reduced spring length). If you don’t mind that risk and want to use 21700, the P42A won’t output as hard as the 40T for the first 15% or so of the discharge curve but will output higher for the latter part of the discharge curve. I have the P42A and 40T. When fresh off the charger, I measure higher lumen output with the 40T so if you are interested in max output to impress people, I would go for the 40T, but for practical use, the P42A is slightly better. The same can be said with the Molicel P26A.

The two well known battery reviewers are Mooch and HKJ.
Here is a list of batteries tested by Mooch.

Here is a battery comparator by HKJ so you can compare the discharge curves of all cells tested by HKJ. You can see the Efest 4200mah is no match for the 40T or Golisi 4300mah.