And 3090 shows you what they can do by unlocking anything on 3080 - it's +10% at best due to power constraints.
There are likely to be GA102s that can't quite make 3090 specification, but will have enough SMs functional so that all 7 GPCs can be used. 20GB or 18GB will also save some power, compared with 24GB, though that power saving will be pretty small.
AMD has to decide how much of the +13% of 3090 versus 3080 it wants to capture. 5700XT is 14% faster than 5700XT which is achieved by a combination of 10% higher "boost clock" (or 8% higher "game clock") and 11% more WGPs. This would require AMD to have shown "72 CU" performance yesterday. While I think that's possible (and a 2.2GHz boost clock coupled with 72 CUs is theoretically 2.1x faster than 5700XT) I am dubious.
RDNA2's memory bandwidth efficiency, due to the 256-bit bus rumours, is a complete wildcard (and mucks up theoretical comparisons with 5700XT). If it's a radically new architecture, then we can assume it's going to take AMD about a year to work out how to make it run well...
If AMD decides to go for 3090 with 6900XT (XTX? water-cooled XTX?), it seems likely that power consumption will be very high. The 3080 20GB is rumoured to launch in early December. NVidia will have about a month to decide how hard this card fights back. So AMD has to balance launch price for 6900XT against its demand.
Ryzen 3 has demonstrated that AMD will not be shy about raising prices, so 6900XT is likely to follow that pattern.
There has also been a rumour of a water-cooled 6900XTX but not this year, so AMD may not attack 3090 this year.
Finally, another rumour says that big Navi will only come with the 3-fan cooler. The 2-fan cooler that was shown a while back presumably isn't cool enough for any variant of big Navi.
So AMD looks likely to be able merely to match 3080 on launch day but pushing past that appears as if it's going to be difficult. Which gives NVidia plenty of chance to spoil it.