The hardware is certainly not the issue here. A 5600 should do just fine with 200Mb with all possible checks turned on.
On the command line you can use cpview, this tool will show you very much performance related information, all real time.
Also in the Smartconsole you can see counters when you enable Monitoring on the gateway object.
Once that is enabled, go to the Gateways & servers view in SmartConsole, select the gateway, then in the botom left part of the window click on Device & License information. A new window will pop-up where you can then choose Traffic, select the top interfaces to see what's going in/out.
Regards, Maarten