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I have read that EOS RP uses otherwise the same sensor as the 6D Mark II. However, RF mount has a lower flange focal distance than EF mount. Therefore, Canon has adjusted the microlenses in EOS RP to work better with RF mount lenses that have the rear element closer to the sensor than EF lenses do. I assume the same is true for EOS R as well.

Now, if I happen to have many EF mount lenses and no RF mount lenses, and am using either EOS R or EOS RP solely with the EF mount lenses, does the different microlens design negatively affect image quality when compared with using the EF lenses with 6D Mark II? So, in other words, when optimizing for RF lenses, does it "unoptimize" for EF lenses?

I have checked the usual sources and DxOMark doesn't seem to have recent cameras such as EOS R or EOS RP, but the-digital-picture image quality tool seems to have EOS R but not EOS RP. I don't see anything wrong with the image quality of EOS R. So, if my assumption about the microlens adjustment for EOS R is true as it is for EOS RP, the image quality should be acceptable.

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    Acceptable is a moving target. What might be acceptable to you might not be acceptable to me, or vice versa. What my be acceptable to you for one image may not be acceptable to you for another image. – Michael C Jul 1 '19 at 15:04
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If, by "image quality" you specifically mean peripheral illumination reduction at the edges and corners of the frame (often called vignetting, though that is a bit of a misnomer for what we are describing), then yes, there will be a slight reduction in IQ.

Whether that is acceptable to you or not all depends upon your definition of "acceptable.

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It seems that the state of the art in DSLM lenses is making them as image-space telecentric (meaning you don't curveball light rays halfway across the camera throat. Perfect counterexample: A pinhole camera.) as possible.

A sensor optimized towards short-backfocus lenses ONLY would be a step in the opposite direction - if it couldn't deal with the rear element farther away, illuminating the sensor in a potentially MORE parallel path, it couldn't deal with an image-space-telecentric lens very well.

If the adjustment makes the sensor LESS tolerant to mildly non-telecentric lenses, it could indeed mean some IQ degradation especially with film-era lenses - however it appears like an unneeded adjustment, given that DSLR sensors need to be compatible with very long focal lengths that will make all light strike the sensor in a rather parallel fashion.

The opposite configuration is known to be potential trouble - lenses with a very short backfocus without any optimization (eg film era, non retrofocus, 28mm rangefinder primes) are notorious for literally very colourful side effects when combined with some older generation mirrorless cameras that reuse unmodified sensors intended for DSLRs. The MFT system always specified that lens designs should be image-space-telecentric to avoid exactly these kinds of issues...

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