Config file is working for me, can you try again and confirm? It's persisting through restarts as well.
EDIT: Did more testing and now it stopped working. To help confirm it WAS(?) working:
1.25 Quake/Source sensitivity
1.238000 MCC Config sensitivity
Using the 360 test on Kovaak's sensitivity matcher I was able to get a near perfect 360 after editing the config and eventually somehow getting it to "work". Changing the in-game sensitivity to 1.2 resulted in undershooting the 360. Changing the sensitivity to 1.3 resulted in overshooting. This would confirm the config change worked, right?
EDIT 2: I found out how to get the config to apply.
With the game closed, edit your config. I did all 7, not sure if it matters. Save the file.
Open MCC, and start a campaign mission. Test a 360, it should overshoot or undershoot. Save and quit to menu.
Start a different mission, and test a 360. It should be perfect assuming you put in your correct sensitivity.
I've tested in a custom multiplayer game, multiple campaign missions and so far it's persisting.
EDIT 3: At some point it reverted back to 1.2 sens even with the file on read-only.