You are correct, RR 640 cannot handle more than 2 drives in RAID, and if those two drives aren't split across the two controllers then even 2 drives can bog it out.
Hope you find something interesting! I'll be keeping an eye on this thread in case you do, I know many are interested! The storage section of XS forum may help you find what you are looking for, but it's down right now for updates so I can't link