I have two keeppower 26650s. I tested their capacity when I first bought them and they both turned out to be even more than 5200mah. However, after giving them average use and not even using them for sub ohming, less than 3 months later I tested their capacity again and they had reduced to 3500mah.
My experience with some MNKE batts is that they were less than stated capacity (they turned out to be 3000mah) but have not reduced. I'm currently using efests but I haven't tested their capacity yet.