The FC water pump housing is aluminium, so it's a weight saving for a start. The FC pump is also less prone to cavitation at higher rpm so is a better unit.
You need to remove some material from the 12A front cover to make it fit.
Not much though. See pic.
You also need to take a similar amount off the back of the water pump housing.
Easy upgrade.
EDIT: the same applies if you want to fit a 12A front cover onto an FC engine to install in an FB using a front mount.