Hi,
FWSM can give you a maximum throughput of 5.5 Gbps under ideal circumstances.
As it has 6 gig portchannel connecting to the MSFC of the switch you can get a maximum of 1 Gbps speed on any port channel at any give point of time. which means any connection through FWSM can get a maximum of 1Gpbs speed provided there is no other connection on that particular port channel which is being used by the connection as FWSM does not load balanace the traffic of a particular connection under any circumstance.
let me know if you have any doubt about this.