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...
Setting memclk to 1100 instead 2100 on RX 480 causes only 20% hashrate drop.
Bandwidth is not a boottleneck at all.
Zcash is not like bandwidth-hungry ETH. There's no 100% memory controller load.