CCD versus CMOS
Back when CMOS sensors were first being made, there was a big difference in the quality of the image versus a same sized CCD sensor. The primary reason was that the actual CMOS photodiodes were much smaller because substantial sensor surface area was for the electronics that processed the signal. The smaller the photodiode, the less the dynamic range because the photon well was smaller. The reasons for CMOS sensors were volume total manufacturing costs, lower power use, ability to read out each pixel separately an higher speed. The cost was less because the camera didn't need as many other components because more of the processing was happening on the sensor - but the initial cost is higher than CCD because the CMOS sensor is more complicated. In volume, the CMOS is less expensive because you don't need as many other chips.
Designing a new camera using a CCD initially costs less, but in volume production, the CMOS becomes less expensive. As time went on, the electronic processing circuitry got smaller and today, if you look at a CMOS sensor versus a CCD sensor, both have pixels pretty much edge to edge. CMOS sensors still cost more to design than a CCD sensor, but the CMOS sensor has the same sized pixels, uses less power, is faster, etc. Today, there is *no* reason why a current CCD sensor is better than a CMOS. Both are made of the same exactly materials and the pixel size is same assuming the rest of the electronics are equivalent.
Different sensors are designed differently so a Leica CMOS sensor may be completely different than a Nikon/Sony/Canon/etc sensor. Leica does not own any sensor manufacturing plants. Leica contracts out all of the sensor fabrication. Some of the Leica current sensors are Sony built and probably Sony designed (I highly doubt Leica has the ability to actually design an image sensor at a basic level) with Leica giving some suggestions. Even the Nikon D850 camera uses a supposedly Nikon sensor, but when you take the sensor apart, you can read the Sony part number. To make matters murkier, the Nikon D850 sensor which was designed by Sony is actually manufactured by a third company called Tower in Israel. Tower makes image sensors for many manufacturers. The particular Tower manufacturing plant is one, if not the, most sophisticated image sensor manufacturing facility. The facility also costs around $10 billion so you can see why one plant gets used for many manufacturers.
The sensor chip is just one critical part in the image acquisition process. There is tons of other processing that happens between the data capture and the RAW file not to mention JPG files. Each manufacturer has their secret firmware processing that happens to impart their particular goals.
With a monochrome sensor, you lose color resolution but gain in spatial and contrast resolution. You can't convert a similar color sensor picture to monochrome and get the same image because the data is different. There are small details that a monochrome camera can see that a color camera can't just as a color camera can see colors that a monochrome camera can't.
As far as converting a color sensor to monochrome, there are only certain models that I convert because I have to figure out various conversion details that are specific to each type of sensor. So I wouldn't want to try a monochrome conversion on a camera I haven't yet tried, such as an M8, unless the customer was willing to assume the risk that the sensor could be damaged or ruined.
When comparing one camera to another, my best suggestion is to compare the stock cameras. If you like one stock camera better than another, then you will like the converted camera better as well. There are so many differences between a D300 and M8, that it is a bit silly to try to compare. What really matters is if you like the images that come from the camera.