I did this for the first time the other day as well.  I managed to successfully supply up to 12GB to 6 of my 32 threads.  I didn't push it any further, no need in my case, but I got the feeling that I could.

I'm curious about what sorts of negative impact using such a large chunk of RAM for render multiple frames might have.   It seems that I push some of my more complicated comps a little too far to the point where AE will almost always fallback to foreground rendering only (not enough RAM available to the background processes), in these cases would there be any reason not to use 8GB, 12GB or even 16GB per background process?