GilP: We have good experience with Keeppower 18650. Bought maybe two thousands of them already directly from Keeppower and all work fine. If I remember right, one battery was dead on arrival, but I would say its good score. I measured capacity multiple times and they are correct.
3_gun: Its little more complicated. Batteries are good quality and poor quality. Good quality ones (Panasonic, Sony, Samsung, LG, Sanyo) have declared capacity spot on and stable performance between all cells. We never ever had any problems with these brand cells and we used thousands of them in our products. Different story is when using protected cells - then it heavily depends on the quality of protected circuit (resistance, discharging, long-term reliablity) and the quality of the bare cell.
Your observation with high lumens probably means that the light is direct driven (so the cell with lesser resistance give more ampers=more lumens) or needs huge current which is too high for normal cells. There are standard cells (<10A) and high current cells. It can make large difference in these direct driven or high lumen lights but thats probably not OP´s case as he needs just 10-1000 lumens.
