As said above, it looks like the 6800xt are identical to the 6900xt except it is a higher binned silicon so it will overclock more stable than the prior release version.
The number of compute units should offer more hashrate but only by fine margins. Much like the 570 vs 580 it all depends on your overclocks used to gain 1-1.5Mh/s more.
The first video of 6900xt I think:
https://www.youtube.com/watch?v=JcZzc8D2cvQ62 to 64Mhs, as you said, only a few Mhs more
So, we have to wait for some bios mod to do some magic on these 6800 and 6900 series, but none of these cards are what we expect, AMD did this series of cards to gamers
Probably will gain a few extra Mhs with miners update, drivers, some tweaks and eventually bios mod, but I expect maximum of 3 or 5 Mhs