There's rumored leaks out of the 390x, and they don't compare to the 295x2 in performance.I'd say that's a relatively sure thing, not a guess.
I'm normally not one for waiting, but I'm definitely waiting to see what the 390x offers especially since rumors have it coming in Feb. with 15%+ gtx 980 performance and having 1/3 the power consumption of a 295x, while likely costing somewhat less than $680.
What is likely to happen is that initially it will release at a very high pricepoint simply because its the latest and greatest, and it will come with some reference cooler that will suck and be too noisy.
Then we'll see better coolers on the market, but still at about the same price, and we'll see some dual-gpu on card versions like the 295x2 for massively inflated prices probably $1400+ as usual.
Then the prices will gradually drop but at this point we're talking about a pretty big time gap between the 295x2 available right now for $615 (or was anyway when I just bought it), and when you'll see the 390x2 equivalent that beats it and is on sale for a reasonable price.
And the 295x2 may have high power draw when gaming, but the reviews say it runs cool and quiet, it just happens to be exhausting quite a bit of heat out the chassis when gaming. I don't see that affecting my electrical bill much, as it does have "zero core" technology which means that when its just chilling and not on full boil it actually shuts down the second GPU/memory crossfire setup entirely most of the time.