I got a chance to install 2 of the cams during the week...
They are running at the same resolution/FPS as I mentioned above. The only thing I have changed is that they are running on their own network via separate Nic in my server.
Running the web client in multi view the CPU usage is around 10%, when viewing one of the cams it goes up to around 18% , clip playback is around 23%.
I think it's going to be OK considering I still have the pfSense VM running on the server.