In most cases - unless you get a bad chip - a Vega 56 can be undervolted and overclocked to run at the same speed as a Vega 64, with very little effort. BIOS flashing probably isn't necessary, IMO; you can most likely do it within Wattman. The two models are very close to each other as-is.
In fact, you could be like me, who started out with a Vega 56 that undervolted ridiculously well, and could be overclocked to a 64 no problem... and then upgrade to an actual 64, only to lose the silicon lottery and discover that the 64 actually requires
more voltage to run at the same speeds that my 56 could.
As mentioned above, the 64 probably isn't worth the extra money. Especially if you're only gaming at 1080p.