ASA5540 firewall throughput is 650Mbps, 3DES/AES VPN throughput is 325Mbps.
Is the firewall total throughput 650M+325M? Or 650M?
650 is the total amount of aggregate throughput the device will support.
The 325Mbps rating is simply specifying that the device can process 325Mbps of 3DES/AES encrypted data. This data would fall within the 650Mbps limit.