So I have a similar issue.

I have 4 bonded links in 802.3ad on my DSSv6 system with raid 6 configured on the 8 drives via the 3ware console.

I have tested across these bonded links to my 2 xenserver hosts and from within a VM I'm getting 200mb/s (mbits not bytes) write and 650mbits/s read, so not even using a full 1Gb link. To ensure there is no problem with the bonds or the switch I have another interface on my Dssv6 linked directly to one of my xenserver hosts and I get the same speeds. I've even tried to use mpio across the bond and the single ethernet and the speed remains the same.

Surely I should expect at least 1gb/s write / read minimum.
Disks are datacenter (I think they call them ES, but cant remember) 1TB 7200 sata drives capable of 3Gbps