Image quality (noise) test results have been added to the Nikon Z 7 page.
Here are a couple of comparisons to get you started:
Nikon Z 7 compared to Canon EOS R
Nikon Z 7 compared to Sony a7R III
Note that these images were processed in their respective brand's RAW converter (Sony-blessed Capture One Express for Sony was used for the a7R III results). With a sharpness setting of one (our current Nikon and Canon standard), the Canon appears undersharpened.
The Nikon Z 7 is in stock at B&H | Amazon | Adorama | WEX (preorders).
Great deal: Save up to nearly $700 on the Z 7 at B&H used, including an "Open Box with full manufacturer warranty" option with $400 savings (I took this option to get the test camera).
Rent the Nikon Z 7 from Lensrentals.