I always go to the DPReview studio comparison page to answer questions like this and actually download the RAW files to pl,ay with myself.
Now the studio shots are not 100% representative of real world results because of the levels of illumination, basically the dimmer they scene then more the differences will show up. So for your Astro images it will be a bigger difference.
As for the RP to 7D MkII high iso performance, the sensor is basically the same as the 7D MkII so the 1 stop advantage is coming from the increase in sensor area as the second link illustrates. RP + RP cropped + 7D MkII.
You can emulate the effect you are going to get by stitching two 7D MkII shots and presenting them at the same output size.
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