If I understand you correctly, you have 6 processors feeding 6 walls that are configured in a 3x2 arrangement...
I just created a test piece of content 3360x1360. I used my 2010 12-core mac pro running Catalyst with 2 RAID0 SSDs outputing on a quatro 4800.
I was only able to run 1 layer of content without frames dropping.
I did however split the piece into two pieces at half size (3360x680) and was able to run them both in perfect sync on 2 2008 mac pros.
If you do the same thing then you'll be fine.
Oh and please share more about the wall. I am impressed by a wall that is 20x8 meters at that high a that resolution.