Which graph? what is it measuring?
You can put more than 5.5.Gbps into a FWSM, doesn't mean you're going to get more than 5.5Gbps out.
The 5.5Gbps figure quoted is for 1500-byte frames -- are you running jumbo frames?
(its throughput is limited by the number of PPS, not the total bandwidth)
The Graph is IF-MIB:ifHCOutOctets based on OID 126.96.36.199.188.8.131.52.1.1.10 we are not using Jumbo frames and the MTU on all interfaces are set to 1500.
Hope this helps and thanks for the reply.