It's not exactly the same setup, but I'm using my M1 iMac side by side with a 27" 2560x1440 display (same size&resolution as 27" non retina iMac). Scaling is fantastic, since objects on the M1 screen are physically the same size as on the external display (same logical 109 PPI). I can see the retina/non-retina difference, but for the most part it doesn't bother me. I use the retina screen for text.
As you can see, my desk is just a bit too small to fit 2x27" displays, so the 24"+27" is perfect.
I also have a late 2013 21.5" non-retina iMac with screen yellowing and dust stains in the bottom corners. The M1 screen blows the old 21.5" iMac away.
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