I just went to hdmi.org. FAQ states here. Does HDMI 2.0 replace HDMI 1.x? No, HDMI 2.0 is built on top of HDMI 1.x and any device that wants to implement HDMI 2.0 must first implement HDMI 1.x as a baseline requirement. Is HDMI 2.0 backwards compatible with HDMI 1.x? Yes, all HDMI versions are fully backward compatible with all previous versions. Does HDMI 2.0 require new connectors? No, HDMI 2.0 uses the existing connectors. Does HDMI 2.0 require new cables? No, HDMI 2.0 features will work with existing HDMI cables. Higher bandwidth features, such as 4K@50/60 (2160p) video formats, will require existing High Speed HDMI cables (Category 2 cables). Can my existing HDMI 1.x devices be upgraded with HDMI 2.0 features? Currently, there are no provisions for doing such an upgrade. Because of the new enhanced feature set, any such conversion would require hardware and/or firmware upgrades. If there are such conversions, it would come from the manufacturer. Please check with them directly. (Some Sony TVs upgraded firmware to support HDMI 2.0 from HDMI 1.4.) It is clear that HDMI 2.0 connectors and cables remain the same. No change. Does mac pro hardware support HDMI 2.0 or 4K@ 60 Hz? All I can say is, it has dual GPUs and HDMI port is on Thunderbolt bus 0, which is cable of driving throughput up to 20 Gbps. HDMI 2.0 specification defined a new, more efficient signaling method, for speeds above 1.4b limits (10.2Gbps), to allow higher bandwidths (up to 18Gbps) over existing High Speed HDMI Wire Cables. Does it become clear to you that new Mac Pro will support HDMI 2?
I read in one of the reviews of the nMP that it only has hdmi 1.4 which is a pretty big issue considering 1.4 only supports 4k at 30hz.
I wouldn't bet on the HDMI port supporting 2.0. It's not impossible, but it's probably unlikely. However, at some point DisplayPort 1.2 to HDMI 2.0 adapters will probably show up, which should let you get a 60 Hz 4K HDMI signal.
It goes on to say it would require a hardware and/or firmware update from the manufacturer. So it is possible. I hope all Macs that have both Thunderbolt 2 and HDMI (currently nMP and rMBP) get firmware updates for full 4K support across both ports. I am a little doubtful it will happen, but I still hope it does.
HDMI on the nMP comes from the AMD card. I think based on this comment about lack of HDMI 2.0 in the R9 AMD cards, there will certainly be no HDMI 2.0 in the nMP... http://www.anandtech.com/show/7400/the-radeon-r9-280x-review-feat-asus-xfx/3
AMD makes the GPUs - not necessarily the HDMI interface. Many Sony 4K TVs shipped before HDMI 2.0 was finalized in Sept 2013 were upgraded from 1.4 to 2.0 by firmware change. Can or would Apple do the same?