Update:
after the test as described below,
I did the first field production yesterday,

under Windows 10.

Smooth as anything!
Tip, do a CLEAN install of Win10 (I upgraded an earlier build to build 10041, which gives a little bug on Win shutdown).

Cheers all.

+++ +++ +++

FYI,
did a test run on a clean Win 10 install, vMix 15.0.0.68

GTX 960, Thunderbolt with BMD Intensity Extreme (HDMI in).
4 GHz cpu, 16g ram.

Fullscreen OUT, IRP, Delay, 1 screen capture, 4 overlays, external mic, flv AND mp4 recording, streaming to YouTube @720p:

smooth as pie.

OK, just one minor thing,
not sure if it only happens under Win10:
Stream - View Status: the window with the "DOS box" look does not show the bottom part, the table with drops etc.
You can scroll down to it, but it jumps back to the top part right away.

You're more than welcome, Richard ;-)