Will a 16340 make my CR123A light brighter?
This is a bit of a complicated question and the answer is both yes and no, depending on the model of flashlight that you have.
Newer Fenix flashlights like the PD25, CL09, or RC09 are designed to be compatible with both CR123A batteries or with 16340 batteries. However, older flashlights like the PD22, P1D, or P1 are not designed to use 16340 batteries and therefore these batteries should not be used in them. At best the light will work normally or at worst it could damage the circuits in the flashlight.
It is important to remember that a 16340 is a different type of battery than a CR123A. Even though they are the same size and shape they are as different as AA are from AAA batteries. 16340 batteries are more powerful than CR123A batteries and therefore could damage the electronics if they are used in a device that is not designed for them.
Always check the user manual of your flashlight or other device to see if it is compatible with 16340 batteries before trying them.