More details about the spec are here: http://www.hdmi.org/manufacturer/hdmi_2_1/index.aspx

Looks like it maxes out at 4k@120Hz

The bandwidth for a higher refresh at 4k is obviously available. But all of the information available so far points to a maximum of 120 Hz at all resolutions. The actual spec is not available, yet, so there's that.

From http://www.hdmi.org/manufacturer/hdmi_2_1/index.aspx

  Q: What are the support resolutions and frame rates?

  A:

    4K50/60
    4K100/120
    5K50/60
    5K100/120
    8K50/60
    8K100/120
    10K50/60
    10K100/120