There are many factors that affect the maximum throughput, from CPU performance to network card drivers and the number of rules. It is best if you test the software in your environment to see what can be achieved in your particular case. Generally you can expect up to 300 Mbps per server, and this cannot be parallelised.
Having said that, in the latest version 3.2.11 we have done a number of optimisations and this version is expected to be able to handle up to 1 Gbps. This is, again, subject to the number of rules and provided bridging is not used, so, again, please test first before purchasing the update! You can check the changelog and download the free trial of new version on the
Bandwidth Manager home page.