The main reason you want to flash the bios is because you can't raise HBM voltage otherwise, so if your particular GPU had actually a good sample of HBM you wouldn't be able to make most use of it otherwise. Anything else can be done with powerplay tables.
Generally if you can get to 1050 Mhz on the HBM then that's good. After that there's very little chance of more & the scaling is meagre, i.e. don't waste your time; bag the 1050 and game happy. Make sure to test for stability in various games, as OCs can be very deceiving - I had no issue until I ran Odyssey then I had to dial it back now it's rock solid. So if you use something like 3DMark don't assume stability there will hold in real world scenarios. As for core it's mostly gonna depend on your ability to tolerate the fan noise. Since you want to put an AIO on it you can probably push it to 1700 mhz (real clock) but don't be too sad if you sustain around 1650 mhz either. The performance difference is again going to be minor. And also, the sweet spot is around 1600 mhz. As you push past that you're gonna sky-rocket the power draw required.
Lastly, no V56 isn't better than V64 but the difference is negligible because no game ever properly uses the full potential of a Vega card anyway, so it comes down to silicon lottery. If you want to make the most use of your Vega then the best way to do that is to push higher resolutions, since that actually does use it better. And finally, voltage behaviour is different at 4K compared to below 4K, so the OC settings won't translate across resolutions exactly.
