It all depends. If the transport mode is, and must be, NBD then yes, you'll need upgrade the management interface speed.
I would advise you to explore the other options for transport mode. "Hotadd" will mount the client VMs corresponding datastore files to a VSA running in a VM. This VSA is its own MA technically, but will typically send the backups to a larger MA where the DDB is hosted. The network traffic will go over whatever network interface is configured for the VSA VM.
If you have a Fibre Channel SAN you could also deploy a VSA on a physical host and use SAN transport mode, if you're comfortable having a Windows host access to the LUN(s) hosting the datastores. I think it works the same for iSCSI, never tried it.
There's extensive documentation available on the various deployment methods and transport modes but with your current setup, 300 GB/h is about as much as you can expect from a single hypervisor host.