Those Aerocool fans don't push that much air.
You don't need a bigger case, and the Phantom 820 pretty much gives less option in terms of water cooling. Your main issue here is the fact that the Zs are turds that push a **** ton of hot air directly in your case rather than out the back. You need ample air flow in and out to maintain good pressure and airflow while keeping heat in check. Pretty much you need to have 3 fast fans in the front pushing air in, and 3 fast fans pushing air out in the top with the H100i exhausting in the top.
Honestly, it'd be much easier to simply ditch the cards and go with something else like 2 980ti or deal with what you have and get 2 Pascal cards next spring.