As an update to my last post on weird Ubuntu 12.04 NFS4 server load: repairing the ext file system actually didn't really work.
Redid the test, problems came right back.
Next thing I did was turn the root volume into xfs: better, but still writing 1MB/s, with 50% i/o utilisation for the root disk.
So perhaps a Linux kernel things. Created a 13.10 nfs server, problem disappeared.