I've got two gigabit ethernet interfaces bonded in CentOS 5.6. I've set "miimode=1000" and I've tried "mode=" 0, 4 and 6. I've not been able to get better than 112MB/sec, which is the same as the non-bonded interfaces.
Most people misunderstand bonding/link aggregation. A single conversation travels over one link to avoid tcp reordering, link aggregation helps with multiple conversations if setup in non FT modes only.