My personal view on this is the following, but this is my personal view.
NVIDIA TitanX was coming, always was but expected much later and now its being sprung upon us and the price could be half decent, especially if around £799 inc. vat in the UK.
I feel that maybe AMD have had a change of heart, realised that most board partners have sold their 290 series stocks of at cost or below to etailors who have also sold 290 series at low margins to hit all time lows of sub £300 for an 8GB 290X, well at least at OcUK.
As such AMD have realised they have a product they can launch far sooner as now it won't hurt sales even if they launch 390X at 290X pricing as 290X is £500 area and I expect 390X 8GB HBM 4k Edition to be around £600 and worth every penny for the outrageous performance it will offer.
NVIDIA have probably gotten wind of this and even though TitanX is also a mighty card I feel the TitanX and 390X will trade blows with TitanX upper-hand being in compute and the additional 4GB VRAM buffer. As such NVIDIA need to get TitanX to market first, to well be first and grab the early adopters and at anything under £800 its a great buy. But I suspect 390X will have similar performance and cost less, where NVIDIA will potentially own the AMD card is on power consumption, but me myself as an overclocker and gamer, well power consumption is no concern to me, that is why we sell 2000W 8 Pack power supplies.
What excites me more is the potential of NVIDIA releasing a new TitanZ with a pair of TitanX onboard with 12GB as 6GB VRAM per GPU would be acceptable and that would be insanely powerful and around £1500 which is same as current TitanZ pricing which sells fine.