Your right, and 390x and 380x will be 28nm as well... which sucks.
However they will have new memory technology (rumors say).
Memory bandwidth does effect performance (even if it doesnt effect "processing power"), it can drastically effect performance esp at higher resolutions (something memory size, ei 12gigs of ram, wont do (unless bottlenecked by it, which even with 4gb now you arnt going to be for like 99% of all games out)).
the 12 gb of ram is just PR non sense, people see a big ram nr and go "ooooh it must be fast" because it has alot of ram. Which makes no sense, is all im saying.
And yes I could see the 390x being alot cheaper and out performing the new Titan card.
Which to me would make it a bad investment, because its likely to drop in price pretty fast.
I wouldnt recammend buying the 390x either though.
The card to get is the ones that come after 28nm.
once you go 16nm + Finfets, the effects from 28nm->16nm+FF, is massive.
THAT along with the new ram technologies will make all cards at 28nm look really dated. Even this new titan card.