As mentioned in TR-4130, Encryption at rest by NetApp Storage Encryption(NSE) support AES 256-bit. NSE requires Self-Encryption Disks and key management server. Data ONTAP 8.1 and later support NSE. http://www.netapp.com/us/media/tr-4130.pdf
For protocol security, as you told from tech support , 256-bit key support of AES encryption for kerberos-based communication starts in Data ONTAP 8.3.