I want to create a local 10g connection between my primary workstation (Windows 10 host Debian 9 guest) and a local native Debian 9 distributed build server. Since Mellanox ConnectX3 cards are now supported in Debian 9, I thought I could just pick up a pair of MCX311As, connect them back-to-back, et voila, the network would no longer be my bottleneck. 
Before I dive in, I am attempting to prognosticate the real world throughput I'll get (given both cards are installed in appropriate PCIe slots with adequate lanes and I use a good cable) on the Debian 9 guest running under VirtualBox. Can anyone provide any information on this matter?
Thank you!
p.s. I posted something similar on the Virtualbox forum: virtualbox.org • View topic - 10g (Mellanox) PCIe performance on Linux Debian 9 Guest, Windows 10 Host