Default [Q] Bootloader

Are the last three alphanumerals of the build number in settings>about the bootloader version?
e.g. Build number: JSS15J.N7105XXUEML3 >> bootloader is ML3

Is ML3 bootloader version 4.3.x?
I am thinking of installing this rom: http://forum.xda-developers.com/show....php?t=2761937 and there's a warning that 4.3.1 bootloaders require AGNI kernel to work properly.

What are the pros and cons of flashing a new bootloader?
I read the guide at http://forum.xda-developers.com/show....php?t=2747963 but it seems upgrading to newer versions are mostly to prevent errors for stock roms. Are there other pros and cons?

Are kernels packaged with new bootloaders/roms or are they separate things (like simply bundled together but can be flashed separately)?
Also, do kernels need to be flashed before or after the rom that requires the kernel?

Much thanks!