A forum member of the Asian ChipHell forum flashed his card with a Radeon RX Vega 64 BIOS and claims that this boosts the performance to a level that's just 2 percent slower than a Radeon RX Vega 64.
The BIOS mod unlocks higher frequencies, a new voltage curve, and enables a higher TDP. It's not 100% certain at the moment whether it's possible to unlock extra CUs by modding the BIOS, the GPU-Z screenshot suggests not but it could be wrong.
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.