Some quick replies:
1. For memory amount, basically, no. Even 4GB will probably be enough for the foreseeable fair lifetime of these cards. Even today they don't fare well at anything above 1080p and this will not improve. At 1080p, you will not benefit in the least from mega texture packs a-la Doom's 5GB requiring Nightmare mode, the eye simply can not tell the difference in texture quality at this resolution beyond a certain point -
and this has been verified by people like Digital Foundry. Bandwidth on the other hand will always be an issue, some games like Far Cry Primal are already very dependent on fillrate and bandwidth, and it will become more critical as your desired antialiasing method and level increases.
2. 1060 can not catch up in DX12, that's a dream.
The Pascal architecture is basically Maxwell shrunk to 16nm finfet, meaning it is critically lacking in hardware schedulers. This is why it is so power efficient, but also why it is so terrible at parallelized tasks and low level API async compute operations. This will not improve. This will likely continue to be the case for Volta, which is very likely going to be Pascal (Maxwell) with HBM.
3. I suggest you watch
this from the 16 minute mark onwards. It will not get better for the 1060, expecially once Nvidia catches on with the times and produces a new architecture with adequate hardware schedulers to take on DX12/Vulkan, when the Maxwell/Pascal(/Volta) cards will become greatly gimped.
4. RX 470 4GB will probably be an undisputed 1080p Price/Performance king. This will come at the expense of around 20% performance and lower memory bandwidth. What that means is the RX 480 will likely keep you more comfortable for about a year longer than the RX 470 would, but both will be great at 1080p.
5. Don't worry about this shit, like, at all.
6. In my mind, there is no reason to buy a Gsync monitor unless you are terribly invested in the nvidia ecosystem with shit like Shield and whatnot. But then, why should you be? Freesync is an open standard, comparable monitors are significantly cheaper and by investing in it you only force nvidia to cave in and adopt it.
7. Never owned a GPU with a backplate, never understood the appeal. Some people are incredibly anal about the way the inside of their cases look, I can't be assed, like, at all. Even though mine has a window and shit, I just go for a practical bang for buck approach with everything. YMMV.
8. I don't believe the 8-pin PCI-e connector was around in 2010, so your computer may lack that for custom 480/1060s. These cards don't draw very heavy loads on the rail though, so the 6-pin connector can be converted to an 8-pin with a cheap adapter and would be sufficient. People will tell you to upgrade your PSU but fuck that, if it's a branded decent CPU that 6-pin should be able to handle the current.
