I have another question, why are MFDBs relatively poor performers at high ISO? Whatever advantage they have they also be able to keep it at higher ISO.
The factors I see are:
Most MFDBs are said not to have variable pre-amps, so the ISO settings above "base ISO" are just "fake ISO". This seems not be the case with then new P65 plus, however.
MFDBs normally don't have microlenses. Microlenses don't play well with lens shifts to my understanding.
The CGA may use narrower filters. This could result in more saturated colors, but not necessarily better colors.
OK, another little tutorial on how to read DxO data:
DR is limited by the weakest link in the signal processing chain (much like MTF is limited by the poorest performing component in the optical path). The sensor has a DR, the ISO amplifier has a DR, and the A/D converter (ADC) has a DR. Ideally, the electronic components downstream of the photosites in the signal processing chain would have a DR as large or larger than the DR of the photosite array, otherwise there is information captured by the photosites that is lost along the way by the noise contributed by these other components. When the ADC's DR is smaller than the photosite DR, one has to choose what part of the photosite's range is to be delivered to the raw data. One achieves this by varying the ISO gain -- larger gain amplifies the lower end of the photosite DR to fit it into the DR window of the ADC, less amplifiation fits the upper end in.
Thus if we look at the DxO (engineering) DR plot for say the Nikon D3, D3s, and P65+ ("screen" tab, so pixel DR)
we see that the the D3s has a very long flat part at low ISO. This is occurring because the amp/ADC DR is about 11.6 stops, and is the limiting factor in the low ISO DR. Increasing the ISO slides that DR towards the lower end of the photosite array's DR, and one hits that lower end about ISO 3200 or so, at which point the DR drops by about a stop for every increase in ISO (because increasing the ISO by one stop pushes one more stop of highlights past the saturation point of the ADC's range). In fact we can deduce from the fact that the D3s DR at ISO 12800 is a bit over 8.2 stops (according to DxO) that the photosite array must have a DR of over 14 stops (since by ISO 12800 one has thrown away the top 6 stops of range of ISO 200 in terms of absolute light intensity, and 8.2+6=14.2); the low ISO engineering DR of the D3s is severely limited by the amp/ADC DR. (BTW, the kink in the P65+ plot is due to its use of pixel-binning starting at ISO 800.)
On the other hand, the D3x and P65+ have a DR that starts dropping by one stop per ISO almost immediately. This indicates that the DR of the amp/ADC is about equal to the DR of the photosite array, so the components are well matched (for the D3x, this is likely related to the massively parallel on-chip ADC of the sensor architecture). This is what we would want in an ideal world -- none of the sensor's range is being compromised by downstream electronics. IF the downstream electronics has a DR that exceeds the sensor sufficiently, then there is no reason to offer variable ISO gain in the camera -- you've already captured the entire sensor range at base ISO, and can achieve higher ISO by simply telling the raw converter to push process according to metering. In a well-designed camera, ISO should be metadata -- and it is on many MFDB's.
A fairer comparison for ISO performance normalizes for the number of pixels in the frame; if we do that (DxO's "print" tab), we find
and the P65+ has with their normalization about 13 stops engineering DR in a fixed size print. The D3s/D3x are in that ballpark at their base ISO (the D3s a bit less due to the poor amp/ADC DR, the D3x a bit more because more of its photosite DR is realized), but their base ISO is higher than the P65+, which means their sensors are more efficient at converting incident light to sensor signal (the so-called quantum efficiency). That means they carry more of that range into high ISO's and achieve better low-light performance.
EDIT: The DSLR advantage at high ISO is rather mostly due to better read noise behavior, not just QE (the lower QE of the MFDB is somewhat offset by the larger sensor area). Perhaps it is better to say that the DR of the photosite array is larger, and by the time we reach high ISO the amp/ADC DR is no longer a limiting factor. The DSLR's simply have better photosite read noise -- about 4.5 electrons for the D3x, about 2.5 electrons for the D3s, compared to about 16 electrons for the P65+. Looking at the graphs again, it is this read noise difference that accounts for the bulk of the over two stop difference in engineering DR between the Phase back and the D3x, and the over three stop difference with the D3s, at high ISO. Again, though, all of this comes at the shadow end; all these cameras collect about the same number of photons over the frame, so the difference is not as great when we impose stricter minimum S/N criteria.