If they can figure out a way to get a hackintosh computer that supports this gpu then I'm selling my imac in a heartbeat.
>90% sure it's not a problem. So far, all new Nvidia GPU (up to TitanX) are supported in the old Mac Pro and Hackintosh by the Web Driver. Can't see why Nvidia will suddenly stop that.
It's certainly not terrible but considering this is a 3 THOUSAND dollar machine and is brand new, this should not happen
$3000 does't mean anything. You were paying that for the 5K screen, the Apple brand, the slim design, but not the spec, the cooling system, upgradability, etc.
Anyway, if you want native OSX support, the Mac Pro 5,1 is your best choice.
Other Mac (including your iMac) is possible to use this card as eGPU via TB, but expect a lot of work have to do, and most likely every OS upgrade will break it.
Hackintosh always has the best cost to performance ratio. But again, not native, easy to have issues during any OS upgrade. Not recommended for inexperience guy to use as daily (working) computer.
For video editing, I am sorry to tell you that the $400 Mac Pro 4,1 with some upgrade may do the job better than your iMac and cost a bit less.
e.g. My Mac Pro's current cost is about
$400 Mac Pro 4,1
Flash to 5,1 (free)
2x HD7950 ($200-300, can be cheaper if buy used card)
$300 1T Samsung SSD
$190 48G ECC RAM
$150 W3680 (or W3690, X5690, X5690, which ever cheaper)
HDD, whatever you want, a WD red 6T HDD which works very well internally is about $200-$250 (you can install up to four of them internally in RAID 0).
So, all together is just about half the cost of your iMac, but sure much more powerful in video editing. However, the best screen to use is just 4K, not 5K in Mac Pro. The 5K monitor works, but may cause trouble, also much more expensive then the 4K monitor.
For dual 7950, you can easily power 4x 4K screen without any problem. 2x Dell P2715Q should be a good choice for 4K video editing. You can use one of the monitor for real time play back at native resolution. Both monitors will cost you another ~$1200 in total.
So, at the end, total cost is just below $3000, not much cheaper than your iMac, but you have 2x4K monitor, 2x7950 for, 48G ECC RAM, 1T SSD + 6T HDD.
Overall a more powerful setting for video editing. AND MOST IMPORTANTLY, this Mac can upgrade, you can use 4xPCIe SSD with ~5900MB/s read/write which only occupy one PCIe slot. You can install a 1080 or whatever GPU to replace the dual 7950. You can go for dual X5690 (12 cores, 24 threads) and 128G RAM (may be more later, when 32G RAM stick exist). For plugin 5x 2T SSD internally (total 10T). Plenty of options in the cMP to fit your personal needs.
Also, Mac Pro is designed to work 100% 24/7, no matter how you stress it, the fan not even run at half speed, quiet and cool.
Please don't get me wrong, I am not saying that the iMac is bad. It's a nice machine, but the same $3000 dollar, you just get a type of Apple computer that's beautiful, but not that powerful and not upgradable (and may not that fit your need for video editing). IMO, the 5K iMac is really good for photo editing, less demanding to GPU and cooling system, require good monitor, more demanding on CPU single core performance (all Mac Pro is relatively bad in this area), less demanding on storage (both speed and size). However, it's not that video editing orientated (especailly 4K video).