Hey, old thread, but I have the option. I have a RX 460. I've been having strain for a long time now and after setting it to 6-bit I actually feel so much better. Gonna give it a few days though before I can say for certain one way or another if it completely resolved my problem, but it's without a doubt better than it was before.
The secret is that you need to use DisplayPort. Using HDMI didn't give me the option to switch to 6-bit, but DP did. Let me know if this works for anyone else!