After the original article, I heard from the Linux game porters at Feral Interactive that the game should actually run with Intel and Radeon if using new enough Mesa, "We have completed the entire game on an AMD machine with mesa during development so it is pretty playable on R7/9 series cards it however is release quality due to some issues with the mesa drivers we are investigating."
In terms of the Mesa expectations, "11.2-git from ~ a month ago from memory, 11.1.1 also drew but I think 11.2 had some fixes over 11.1.1."
Knowing that they had got it working, I went back and tried some other configurations. When using Ubuntu 15.10's default Linux 4.2 kernel while using Mesa 11.2-devel from the Padoka PPA, I got the game loading on a Radeon R9 290. Hoorah!
So with Mesa 11.2 it should be possible, at least for my few minutes of clicking around in the game. They also clarified that the Windows version of XCOM 2 lacks a benchmark mode, so unfortunately that's about it for the RadeonSI testing with no performance measurements to share. So if you swear by the open-source driver, it should be possible to play this latest strategy game for Linux if you are using Mesa 11.2-devel Git and potentially need to navigate through a few hoops; the XCOM 2 discussions on Steam Community are also pointing out a range of bugs/issues being encountered even on Windows. Once again the official Linux launch requirements for XCOM 2 are a GeForce GTX 650 as minimum while the recommended GPU is a GeForce GTX 960.
Update: And here is the Catalyst test results for XCOM 2 on Linux.