easy numbers.
a 5700XT without bios mod, at stock 1750mem does around 51 ish mh/s. At 1850 does 53,5mh/s (without bios mod, obviously). So almost every 80mhz on the memory, you get around 2,5mh/s (appart bios mod which would take it to 56,6mh/s at 1850mhz).
Since RX 6800XT has 16gpbs stock, so 2000mhz stock, at most it will do 60mh/s without bios mod, even considering more efficiency on the memory.
Now, if the card could do at least 2150mhz overclock, (which i don't think it will do), we could be talking about 64-66 stock without bios mod. With bios mod, i think 70mh/s are granted.
The power consumption is the key question. i don't think it will consume less than the 5700XT so all in all considered, and price included, i think this card will not be good enough to justify an investment.
If you factor the rest of issues (DEFY is dead+PoS in 2021+more hashrate on the network like madness), you have to be really mad to buy anything right now.
IMO, only the RX 5700Xt can justify an investment if you can get it around 300-320$ (320€ here in Spain).
This are the numbers. Will we see better performance on this 6800XT?? I don't think so, but we will see.