I'm pretty blah about such normally but I'm pretty curious how this will play out.
I still vote the 390x to be the $500ish at launch high end mainstream-enthusiast card like the 290x was rather then a Titan competitor. That would be more of a coup than I think current tech from anyone would allow. Maybe a higher end card later or a 395x2 or whatever. I'll be amused to be proven wrong though.
I think, based on the tech, the 390x will quite likely compete favorable against the GM200 in the Titan X.
It may or may not have the same compute capability, depending on what Nvidia debuts inside of the Titan X variant of the GM200 (surely the 9## version of the GM200 will be crippled in FP64 compute, aka double precision).
But in terms of gaming performance, the 980 wasn't THAT far ahead of the 290X. Yes, the GM104 is a smaller die, but it is the current architecture for Nvidia, and they can only scale that performance so much between the GM204 and the GM200 full-size die. Whereas for AMD, the 390X is an entirely new generation of architecture, and while not an entire redesign, it is definitely a refinement compared to past iterations. Will they scale 200% compared to the last full-size die they released? I sincerely doubt it. But I would not expect them to only perform 30% higher than the 290X.
If AMD's 390X bests the 290X by 40%, it may not perform better than the GM200 in every benchmark, but if they release it at the original MSRP for the 290X (around $550-600), they may very well take this generation in a landslide, at least until Nvidia can answer with a reasonably-priced 9## variant of the GM200.
I don't honestly expect the 390x to beat the Titan X, but in some ways, it is very much possible.
If it DOES take the performance crown from the Titan X, I doubt it will release at $550. But in no way do I see AMD attempting to compete with Nvidia at the same market level. That would be suicide and, frankly, one of the worst business decisions a major company has made in at least a few decades. Perhaps not that bad, as I'm not at all interested in researching the worst business mistakes in decades, but while it may be hyperbole, it is not outside the realm of possibilities in this scenario.
If AMD somehow bests the Titan X, I think AMD would be wise to release the GPU at $650. It is unlikely Nvidia would be willing to release a 9## version of the GM200 (let's call it the 985) at anything less than $650, and until Nvidia can match or counter the 390x's price, AMD will rake in the money when comparing against the laughably-priced Titan.
Make no mistake that the Titan X, regardless of the competition, will make money for Nvidia simply because of the clout and brand presence Nvidia carries at this point. I'd love for AMD to be able to command the premiums Nvidia gets away with, and while I don't want those numbers at any point, that would at least mean AMD is healthy again. For now, as long as AMD can target around $600 or higher, and actually sell in good numbers, they can meet revenue targets. It is when Nvidia is able to counter with cards priced under $400 that revenue for AMD tanks, at least if those cards are their largest die.