Rick...your post got me thinking about what the actual benefit would be. First off, IMO, IQ in general will increase with more MP. I think specific types of photographers and photography, as you mention, will benefit more from the extra resolution. If pixels were ideal, it would be a linear relationship. But they are not ideal.

If pixels were ideal, then a crop sensor would truly given 1.6x resolution bump. You came up with 1.2x. 1.2/1.6 = 0.75. If the rumored 51.1 MP sensor camera is truly based on the 7DII sensor, then the 51.1 MP sensor would be somewhat equivalent to 38.3 MP if we applied the 0.75 ratio compared to your 1.0x (5DII). So, a benefit, no doubt.

But, as I mentioned above, the reasons crop resolution isn't ideal are both sensor related and lens related. As the lens related issues would no longer be a factor, 0.75 would likely be the low end.

That made me wonder how much of the divergence from ideal is lens and how much is sensor. Which got me playing with numbers. When people report the "pixel size" the calcs I have seen are simply taking the sensor area and dividing it by the number of pixels. So, for the 5DIII, FF sensor area is 36 mm x 24 mm = 864 mm2 / 23 MP = 6.1.

But, that is another "idealized" calc. It assumes that there is no edge to the pixel. Or, using the pixel=bucket analogy, no edge to the bucket. And a bucket without edges doesn't hold water.

I suspect one of the reasons that higher pixel densities are not as efficient as lessor densities is that with higher densities more of the sensor area is lost to the "bucket edge." This may be around the actual pixel on the sensor or maybe the Bayer filter.

So, the reason for my post, I am wondering if others know, is that correct?