If you are preparing the system, and applied the Replication to your system, for sure first time the replication running it would take some resources and bandwidth, but after finishing the replication ( for first time ) you will not notice that. Mostly that's why with your old system ( with V6 ) you did not notice it, because it is already done before time. So please wait tell the replication is done between your systems, then you can compare the performance again.

For your NIC cards, are they supporting the second generation of NetXtreme ? or only the first one? as we already have a small update file for the NetXtreme that support second generation, which should give you a more better performance too, to get this small update, please open a support ticket and our support team will support you with it.