Only 5-7% from my research. Not really worth the extra powerdraw if you ask me. Of course I'm not going to **** on anyway that bought a 6900X though as you do get a very nice shiny Gold box with that one.
The thing is reviews have not conducted controlled testing like i did, so their comparisons are good for a rough indication but are not accurate. Also the power draw is very similar between the 6800 XT and 6900 XT, that's why they both have the same TDP.
The power draw from the extra shader cores and compute units is somewhat offset by the 6900 XT being better silicon, and requiring less voltage for the clock speed.
The 6900 XT draws fractionally more power when i compared Wattage drawn at the wall between the 6800 XT and the 6900 XT. The difference comes in the power spikes, which are definitely larger on the 6900 XT, but the average power draw is similar.
Enjoy your 6800 XT, i miss mine it was a bargain at £600 delivered.