And as an additional node, you DO NOT want your production storage to be the bottleneck, as probably at the point the source will be 100% loaded in reading data, the production VMs will suffer from lack of I/O. One of the reason for example we introduced backup I/O control in v8.
More than pure performance, I'd look at different parameters like backup windows, or RPO. How frequently you want to run your backups? Would you be ok with backups running 4 hrs a day, or not? And from here understand what you need. It may be that the first upgrade to reach your business goals would be the production storage, or the network, even before the backup target. With the new SOBR technology is damn easy to add an additional target to improve performance if the first one is not enough.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1