Sadly, I have never had such requirements infront of me.

Have you considered also GFS2 or even XFS with drbd bellow?
As last resort a Gluster/CEPH cluster can also be tested, although the costs (hardware) will be higher.

I would be happy to learn more about your project.

P.S: you can also add 'nobarrier' mount option if you have a battery-backed cache.