WOOW the change is indeed huge, do you have the render times for those ?
About 3 minutes for each, it was set to pass limit as stopping condition. I didn't look at noise percentage, but MSI 0 was obviously much noisier before denoiser. One would need to make more in-depth tests before drawing any conclusions, but as Aram said, with the new fireflies filter you can now play with MSI. Just a reminder - you want to increase it to make its effect less destructive. Setting it to 0 is a special case, i.e. MSI is effectively turned off, which is not recommended.