The same driver for multiple network cards -- performance bottleneck?
It won't be the driver.
It's most likely to be a physically shared component, such as interrupts or the PCI bus.
Are they sharing the same interrupt (IRQ)? This is probably your bottleneck.
What is the endpoint of your iperf test? If you are routing through network hardware or combining all output to a single GBe NIC on another machine your bottleneck may be remote.
I've posted some sysctl magic here. You can try it, see if it helps
PS. How you benchmarking speed?