Once everything was set up, we put the machine through Geekbench 3 (which, unlike many other benchmark tools we considered, actually has a Linux version). The results show a definite edge for Windows 10, especially in terms of floating point operations. Still, SteamOS is within the same order of power magnitude in many other performance metrics.
No matter how you slice it, running these two high-end titles on SteamOS comes with a sizable frame rate hit; we got anywhere from 21- to 58-percent fewer frames per second, depending on the graphical settings. On our hardware running Shadow of Mordor at Ultra settings and HD resolution, the OS change alone was the difference between a playable 34.5 fps average on Windows and a stuttering 14.6 fps mess on SteamOS.
Unfortunately, Valve's own Source engine games showed the same performance hit when compared to their Windows versions. Portal, Team Fortress 2, and DOTA 2 all took massive frame rate dips on SteamOS compared to their Windows counterparts; only Left 4 Dead 2 showed comparable performance between the two operating systems (though there's no sign of those SteamOS frame rate improvements Valve cited years ago).
Now obviously, this mainly comes down to driver optimisations and the fact that DirectX has been around for so long and is a great API. I'm just not sure if this will suddenly change, seeing as SteamOS and Linux make up less than a percent of the Steam hardware survey.
I'll definitely be going the route of Steam Link instead of Steam Machine.