One must wonder if the Wattman "bugs" with Vega voltages were intentional to prevent this?
Although it was expected, the sample used by KDtree did not unlock to full 4096-core Vega. This means that flashing 64 BIOS into 56 will not give you any extra boost thanks to more cores. However, the BIOS will change clock speeds which apparently have a much greater effect on the performance.
The RX Vega 64 BIOS has 1545 MHz boost clock and 945 MHz HBM2 clock. The RX Vega 56 is slower with boost clock at 1471 MHz and memory clock at 800 MHz. The BIOS mod gives you the same clocks on your 56, which practically improves performance.
By increasing the frequency by 75 MHz the modder noticed that the card was only 2% slower than reference RX Vega 64.
Furthermore, it was proven that overclocked RX Vega 56 at 1650 MHz (core) 2200 MHz (memory) actually surpasses 64 at default clocks.
If those results are correct then RX Vega 56 may become even more popular thanks to this simple mod. Of course, such modifications will void your warranty, but since the RX Vega series have two BIOSes, this mod is almost fool-proof.