My 2cents: increase the number of iperf threads from 2 to 8 or 10 or 12. this will spread traffic better over all CPU cores.
Give it a try, let us know.
i spoke with our performance team - they are saying that they see no special reason not to achieve the numbers you are shooting for.
maybe you are not going to hit the 80G (because of protocol overhead but you should be able to get close to it.