If you're concerned about network contention with NFS, you should use a separate network just for NFS. On an Exadata X2 system, you will have several unused ports. Those can either be standard gigabit ethernet or 10GbE, depending on your configuration. Also, if you're planning on using NFS for backups, look into directNFS (MOS note #762374.1).
If you can afford it look at using zfs over infiniband for the best bandwidth if you go for 73xx or 74xx models. You will also get dual controllers for full redundancy.
Oracle's benchmark backup figures of 7 tb an hour are based on this setup If you go for a standard or 10 gig nfs connection you will get considerably slower speeds.