I have a 6900XT though currently running with a 3600X, there is a newer 6900XT GPU out there (well 2 actually) that most board partners are adding the ultimate label to it. These cards clock higher and seems to be able to edge the RTX 3090 out as long as you are not using ray tracing or DLSS (if your game supports it).
One thing to consider is that Nvidia cards do not have a scheduler on them so it uses more CPU resources to run then AMD cards, meaning if you are in a CPU limited situation then the AMD card will edge ahead of the Nvidia card by a good amount. Though the 5900X or even a 5800X as well as Intel 10th gen and 11th generation CPUs with probably 6 or more cores should be fine here, though something to take into consideration depending on full system specs. The 6900XT caps GPU clock speeds to 3000Mhz and 2150MHz on the memory, the 6900XT ultimate will go to 4000MHz though this is only useful on LN2 cooling these cards will clock higher at the same voltages.
Will be interesting to see what Nvidia and AMD bring to the table next year as well as what Intel can cook up.
edit: the card above this is an AMD only 6900XT liquid cooled card that is currently only being sold to OEM which allows for increased bandwidth for the DDR6 video ram.