iperf on Windows doesn't perform very well in general, the results seem quite good regarding that already. You can play with the settings a bit more until you've hit a sweet spot, but it's hard to tell. Try -P 10 and -w 65536 for more parallel sessions and an increased TCP window size.
Also make sure the VMs you use for testing have enough vCPUs to handle the load you're pushing through the virtual wires. Jumbo frames could surely also come in handy.