Sony HX200 - 18MP issue

I think he was talking about average bits/megapixel in JPEG files, causing artifacts (but not the bits per primary color).

Whether the JPEG bit/megapixel ratio that the camera uses is better when selecting 16:9 needs to be tested for your specific camera, since this could be firmware-version dependent.

I too noticed in WX50/WX70 samples that the JPEG compression is slightly excessive in 16MP when viewed at 100% (in addition to Sony's noise cleaning algorithm). However, after downsizing to 4MP in Photoshop, the few JPEG artifacts present become invisible. Just ensure that you save the result in a lossless format such as PNG, and not as JPEG again!

On the other hand, consider that using only 75% sensor area could hamper the noise reduction efficiency of the post-process downsizing (because Photoshop will have fewer pixels to play with).