As long as I use the right right cables and graphics card, will this support higher refresh rates than 60hz on QHD 2560x1440 single monitor? Or will it be limited to 60hz even on QHD monitor?

If using a system with DP 1.4 output you can get higher refresh rates than 60hz on QHD 2560x1440, but the second display will get lower resolution & refresh rates. MST is share bandwidth, when one get higher another one will get lower. If your system has DP 1.2 output you will not get more than 2K@60. So the resolution and refresh rate depends on your host system's DisplayPort output