From what I've been able to gather, Supersampling in the settings is just for models to make them smoother. HMD Image Quality will increase the the actual display super sampling. I've found that Turning in game Supersampling down to .75 and HMD Image quality to 1.5, decreases the amount your graphics card has to work to render objects but greatly improves readability of text on screen. Try it out.
Also if you're using a VIVE, I would suggest updating SteamVR to the Beta and enabling Asynchronous reprojection, disabling interleaved reprojection, and enabling always on reprojection in the SteamVR settings. These settings are currently only available on Nvidia cards BTW.