Will I notice a difference if I add another h/c to my current setup?
 

I currently only run the cpu block (maze 4), mag 3, and one heatercore, approx. 9.5x6.5, with four sunon 84 cfm fans on push-pull in a 15"x9"x9" box i made for it all. I was planning on adding a gpu block, and redesigning the box, with a 1" shroud for the fans on either side, and i make it so an identical heatercore could be mounted on the opposite inside wall of the box. Would this help my temps any? I'm currently getting 38-44c on just water, and 13-20c with the pelt. I don't have any means of measuring water temps right now, ambient stays cool at 75f/24c, but will another heatercore help overall temps any? :shrug:

Quite probably. We need to know your water temperature, though.