According to the Cisco product description of the ASA 5550 the appliance has a maximal throughput of "up to 1.2 Gbps". It's pretty common for Cisco to measure throughput in interesting ways. Mostly they'll add up RX and TX and mention the combined bandwidth. In this case this means that you're likely to be capped at somewhere around 600 Mbps of bi-directional traffic.
One thing you really have to watch out for, to get more out of that particular appliance, is balancing the traffic across its two slots. You can issue show traffic on the CLI which gives you the current balance at the end of its output. If you're just using one of the two slots and your CPU usage is at 100% you'll experience packet loss despite the fact that there's still some free capacity. It seems that the CPU is blocked too much waiting for the queues to get free at this point, when trying to send back traffic to where it came from. Using both slots the CPU usage went back to 50% which seems much more reasonable.