Bad theory.

 I have yet to see ANYTHING where the performance of the RX 480 matches the RX 390X - even ETH which is very hard on memory (and is probably the point the RX 480 comes the closest to matching the RX 390X).

 ZEC has proven to be a lot less memory-hardened than it's developers appear to have intended it to be - at which point the far superior number of computer cores (abet as somewhat lower clock) of the R9 390x start pulling away from the RX 480.


 Keep in mind - the RX 480 is NOT a high end of "Enthusiast" range card like the R9 390x was for IT'S generation.
 It is targeted at the high end of "Mainstream" or the BOTTOM end of "Enthusiast" cards for the current generation.
 We still haven't seen the high-end *OR* the bottom-end of the "still in the works" CURRENT generation of AMD cards, much less the next-generation Vega to come.