Core issue
When you attempt to create an EtherChannel with two or more ports on a module (for example, WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6148-GE-TX, and WS-X6148V-GE-TX), you receive a message that the EtherChannel bandwidth will be limited to 1 Gbps. This is because these modules do not support more than 1 Gbps of traffic per EtherChannel as per the release notes.
For EtherChannel, the data from all links in a bundle go to the port ASIC, even though the data is destined for another link. This data consumes bandwidth in the 1-Gigabit Ethernet link. For these modules, the sum total of all data on an EtherChannel cannot exceed 1 Gigabit.
Resolution
On the high density 10/100/1000 copper modules, the only modules that are capable of supporting an EtherChannel with greater than 1 GB of bandwidth are the WS-X6748-GE-TX and WS-X6148A-GE-TX.
For more information, refer to the Gigabit Ethernet Switching Modules section of the Release Notes for Catalyst 6500 Series Software Release 8.x.