The main difference between 300 and 200 series is that 300 series did not have a reference design, and on paper AMD upped the memory and core clocks.
For practical purposes, an aftermarket 290X with typically overclocked core out of the box running on 15.7 drivers is likely to be equal to a 390x.
When you look at benchmark comparisons to the old 290x, you are probably looking at a reference 290x bench. The reference boards were notably horrible on noise and heat. This makes the 390x look better than it really is.
This is actually the big fallacy of reviews all over these days. I just looked at a 970 review where the card matched up against a 980 up to 1440p due to OC. But that was a ref 980, and you can get a 20% OC out of the box 980 for $499.
But very few review sites do apples to apples comparisons. I don't think it's in their interest to do so since it means they get to piss of one of the AIBs or GPU makers along with their fan base (whoever loses). This is why most card reviews are positive. It's also why an AIB 290x vs 390x comparison probably won't be seen on major review sites (as someone said, they don't have the balls), and if one does happen to show up it'll probably be using a 4GB 290x at resolutions and textures that give the 8GB 390x an advantage (despite the fact that 8GB 290Xs were/are available).
For the consumer it comes down to the specific card you are looking at in comparison to another specific card if you want to make an informed decision.