To get the full refresh rate of your custom display mode, go to Steam's Library tab, right-click Team Fortress 2 (or any other Half-Life 2 based game, they should all accept these parameters), select Properties.
In the Properties window, in the General tab click "Set launch options..." button. Add
-freq 68 (instead of 68 use your refresh rate). Also, if you've been playing in windowed borderless mode, add
-full to ensure the game launches in fullscreen mode.
Check your monitor's OSD to see if you're running at the expected refresh rate.
In the game's graphics options disable vertical synchronization to reduce output lag for the fastest response time between your mouse movement and the displayed picture.
By default the game's unlocked framerate caps at 300. That's a lot of frames you don't get to see, so you can reduce the noise from the GPU fans by opening console (by pressing tilde key ~, or if you don't have it enabled, also add
-console into launch options) and typing:
fps_max 137. People recommend to set this value to
(refresh rate * 2) + 1 (in my case, it's
68 * 2 + 1) to prevent screen tearing, but still saving otherwise wasted GPU resources, and as a side effect reduce power usage and fan noise. I'm not aware of a launch option command to set this, so you might have to do it every time you launch the game manually.