Techpowerup have it in a chart.
The 7900Xt is 54% faster in 1440p and 4k over the 6800.
AMD Radeon RX 7900 XT is built on the same Navi 31 GPU as the XTX, but with fewer chiplets, which helps bring down the price. In our review this $900 graphics card is able to match the performance of GeForce RTX 3090 Ti that launched at $2000 just a few months ago.
www.techpowerup.com
In ray tracing it nearly doubles the 6800 performance as well in some games at some resolutions, but seems to be 67% faster on average there.