UPDATE: I checked out the Moshi adapter - unfortunatelly there is no any detailed specifications, the Manufacturer's page doesn't even mention that it is HDMI 2.0. The same with Belkin. I think both of these are passive adapters and won't produce anything beyond what the graphics card can offer. Especially since Apple officially supports 4K@60Hz using MST (which is a displayport/thunderdbolt 2 specific thing) while in this case we are talking about 4K@50Hz SST.
VisionTek has active ones but seems like they also don't do 60Hz.