I currently own 3 different 26650 mods: Boxer75, Dani #6 and Presa100. I have 4 different brands of batteries for them, Boxer, eFest, MxJo and Basen.
Sadly all the batteries have very noticeable sag/droop on +30W builds. The Boxer is the worse with the #6 coming in at a close 2nd. I have not noticed droop with the Presa100. (maybe the battery indicator is NOT real time like the others?)
With lower power builds the 26650 mods can hold their own compared to dual 18650 mod's battery life (even sometimes bettering it) however if high power builds are your thing a parallel 18650 mod would be a better usage fit and have better battery life.