Using NDMP option to backup very large netapp volumes. I need a way to reduce my windows so I do not cause week day performance issues.

You sir have run into the limitation of NDMP and large data sets.  There is not much you can do with BE to solve this.  No option or add on from a BE perspective is going to solve this.

You can go to LTO5, but the odds are that the filer cannot do it's UFS_DUMP fast enough to feed those LTO5 drives.  

You can go to NetBackup which will now with 7.01 allow you to multiplex NDMP backups.

Or perhaps look at NetBackup and using synthetic backups with an incremental forever policy.

Lastly, is all of this data, active data?  Perhaps the easiest approach, would be to start pruning the inactive data, say more than 6months old or 1yr old off the Filer using Enterprise Vault?  Then move it to cheaper DAS based storage or similar.  This would extend how much expensive NetApp storage you will need to buy, make backups faster, and reduce your backup windows.  End-users will not know anything has changed, as Enterprise Vault can archive off CIFS and Netapp no problem.