I presume you are referring to LAN based backup here ?
Couple of things you could try before opting for an additional media server are:
1. Ensure the streaming and multiplexing options are set properly. Improper settings can adversly impact the backup performance
2. If network interface [to the production LAN] is the bottleneck, you can look at a dedicated 10G link between the NBU server and the switch [if supported]
I am not sure if you can enable load balancing between media servers with the conventional DSU/Media manager storage units. As far as I know OST LSUs support load balancing. You might want to reconfirm this once.
Could you run something like an iperf between your clients and the master server, to check for duplex mismatch (backup would be EXTREMELLY slow). Have you researched the NICs? We've seen some NICs that just don't work that well, and in other cases, we've seen drivers that weren't working right and that caused a slow down, maybe not enough to show as badly as a duplex mismatch, but quite a bit.