Hi Russ,
You are converting 12x500GB LUNs to 2x2TB LUNs. Without knowing the raid levels and spindles per LUN for the old and new configurations, it is difficult to understand the true impact. For now, let us assume that you are using RAID6 and you were using less spindles per LUN for the current configuration. You are likely to have more spindles per LUN for larger (newer) configuration which in general increases the read performance (backup) significantly.
Virtual machine backups using vStorage APIs are at the VM level. Your old configuration used less number of spindles per VM, the new one is using more spindles and hence the backup performance is likely to increase. But please note that the statement is based on assumptions on RAID level and spindles per LUN.
In NetBackup, the backups jobs per data store (and other resource throttling parameters) are for use with VMware Intelligent Policies. This is where NetBackup automatically discovers VMs based on the criteria you set. That feature is not supported for regular policies, it is not needed in general as the administrator has control on how many VMs to include in a given policy. Are you saying that you have problems in resource throttling when using VIP?
Disclaimer: I work for Symantec. Comments are my own.
Warm regards,
Rasheed