Apple’s drivers should create resolutions depending on the reported (or assumed) capabilities of the display, the DisplayPort adapter, and the graphics hardware. SwitchResX is used to create custom resolutions that the drivers do not automatically create.
Custom resolutions created in SwitchResX are validated by the graphics drivers which may choose to disable any created custom resolutions based on the reported (or assumed) capabilities of the display, the DisplayPort adapter, and the graphics hardware.
If the graphics drivers are incorrectly disabling resolutions that have pixel clocks greater than some fixed value for the port that you want to use, then a patch for the Apple drivers may be required. So you made need both SwitchResX and the patch.
However, if the drivers disable a resolution because of something reported by the firmware of the mdp-hdmi adapter, then a firmware change could fix it without the need for a patch.
SwitchResX can save the EDID for a display. Use the “Export EDID” button. This will contain information about the capabilities of the display.
For each display, SwitchResX shows in the Display Information tab the allowed frequency ranges of the graphics hardware (maybe even DisplayPort adapter - I’m not sure). The max pixel clock frequency should be greater than 533 MHz to allow 4K @ 60Hz. If not, then a patch may be required.
To get the capabilities of the DisplayPort and DisplayPort adapter, then Linux (such as Ubuntu) can be used to dump the DPCD information (this is like EDID for the display but is for the DisplayPort/DisplayPort adapter). If a firmware update fixes your problem, then it would be interesting to know if there are changes in the DPCD that are the cause of the fix. Maybe all the adapter has to do is not report itself as HDMI (if it’s even doing that now?) because it’s an active DisplayPort adapter and there’s no reason for the OS to know what’s on the other end of the adapter (except for the EDID information which describes the resolution of the display, which you can override).