It's true they have upped the memory across the board but that doesn't necessarily mean much or anything at all.
For instance the 390x is confirmed to just be a 290x with 8Gb of memory. That already exists and is shown to have no improvement in performance. If the chipset can only process a certain amount at a time, more memory does nothing but sound nice when advertising.
The 290x was released in 2013 and is being re released in 2015 under a new name. AMD clearly have some huge problems with their engineering team unable to come up with anything new.
Say hello to the first review of the 390x.
http://www.tomshardware.com/reviews/sapphire-vapor-x-r9-290x-8gb,3977.html