Transistors rx480-5,700 million vs 390x-6,200 million
Bus Width 256 bit vs 512 bit
so your 480 is approximately equal to the 280x and will never reach the level of 390
So what ?
Pitcairn have 256 bit bus too, but only 100-120 H/s
Fiji have 4096 bit bus, but it does even less than Havaii.
Bus width is only part of the equation. Bandwidth is a better representation.
RX480 vs R9 390 vs R9 Fury
Frequency: 2000 MHz 6000 MHz 500 MHz
Interface: 256-bit 512-bit 4096-bit
Bandwidth: 256 GB/s 384 GB/s 512 GB/s
Being that equihash is specifically designed to bottleneck on bandwidth I wouldn't hold my breath for major increases on the RX series...