Have you tried using iSCSI instead of NFS ? You can use MPIO, and get more through put this way.
You don't mention drive speeds either. What kind of drives are they, and are you using software RAID, or not?
I don't have ISCSI setup done yet, i will give them a try if this would increase the performance.
Normally my goal was to have around 10-15 machines to run on this kind of Setup (NFS/2x1Gbit) - is this possible per design from a storage perspective??
I created the both NFS pools with each 4GB of cache-space, is this ok? (hole partition size is 900GB each).