Hi,
1) It's $1200USD MSRP (probably will be price gouged at first).
2) 40% more processing cores, but likely won't hit quite the same frequency. With other bottlenecks real advantage might be closer to 20% on average, though this will vary a lot.
3) *It's not primarily a gaming card. It offers advantages due to Double Precision (which gamers don't need) for non-gaming uses.
4) I don't normally recommend SLI, but for the money a 2xGTX1080 would make a lot more sense. Or put the money towards a GSYNC monitor or something else.