Richard, I have my screens just below the visual sight line out the window. Some comment that this gives a "cluttered look"--true. But I can see the chart plotter with my peripheral vision as I look ahead, and when I pull my eyes down to the chart plotter, I have the view out the window in peripheral vision. So I have the two chart plotters/fish finder on the top of the console, and the Radar hung down from the overhead.
They are set up so my wife can look at them also. Generally if one of is is looking at an instrument, the other is looking ahead. Of course in pea soup fog, or potential dangerous situation in the dark, we are down to a displacement speed and all senses are very important.
I consider the worst of the world is a "Console mounted screen"--maybe even worse is the far overhead box mounted screen (some center consoles and large boats have this) (have to have special trifocals to see that one!) These cause you to have to shift the eyes way off of the foreward view. I have used this is large trawlers, but in a smaller fast boat, things happen way too fast for me when you have to concentrate away from the foreward view....
I do a mental overlay of the radar--but I have been doing this for over 30 years, and started doing this with paper charts, so it seems to me to be the best say.
The larger the screen, the easier it is to see the detail. I don't want to be saying "is that the bouy or is that a boat plus the bouy?"--For me it is easier when they are separate--
All of my experience is on other folks boats (larger than C dory) with overlay--and many of these folks do go to separate screens for the radar and chart plotter. With a networked system ($$$$) you can have both--either overlay or separate screesn. But these systems so far have been considerably more expensive. I tend to have a minum screen size of 5"--and really prefer at least 6 to 7 inches. The CD 25 came with a Garmin 176 C which is a 4" screen, and way to small for me. If you have an 8" screen, then you will have 4 small screens which are no bigger than the 4" screen, and most likely slightly smaller.
Try and see various layouts on boats and see how they work when looking at sight lines foreward. We all have different requirements.