I have a 16, and run 2 4k monitors without issue. So couple of questions:
1) do you have the model with the discrete GPU?
2) Have you installed anything like gfxstatus, or other tools (including terminal commands) that override the automatic graphics switching in OSX?
3) Running windowed or full screen?
4) Are you starting the VM before or after plugging in the monitor?
The net is: with a discrete gpu, make sure that there's no override software installed, and automatic switching is enabled, run windowed, and start the VM after the monitor is plugged in
FWIW I did see lag, both for VM's and OSX apps with two monitors running at 4k/30hz over hdmi, but it was usable. I invested in an eGPU and now run both at 4k/60hz over displayport and it's blazingly fast (I can even play dragon age inquisition in a vm now).
