These debates are always fun and interesting. However, on the question of whether M2, M4, V10, or any of the other common high-speed steels used in making turning tools, can be sharpened as keenly as the others, this has been answered in the lab. One of the ways to reliably testing the sharpness of an edge is to use an instrument that measures the force necessary to cut through a standardized, calibrated medium. The less force this takes, the sharper the edge. (See, for example, the Edge-On-Up Edge Tester. There are other brands that use, more or less, this same method.) There are other types of instruments that measure the sharpness of an edge. My point is only that there are instruments that measure an edge's sharpness, and it has been demonstrated through the use of those instruments that all of these tools can be sharpened to same degree of fineness. That is, any differences in sharpness can be accounted for by the variability of one sample from another sample of the same metal, or any differences fall within resolution of the test instrument.
So, why do so many of us believe that one of these steels can be sharpened more finely than the others? I think two things are going on. First, each of us is a sample of one. Our experiences are unique to us. Your M2 gouge is different from mine, even if they were made in the same factory. Each piece of wood that you turn is different from the wood that I turn, even if it is the same species. Heck, each piece of wood that you turn is different from the next piece that you'll turn. Given all this variability, is it any wonder that we might have different impressions of how sharp an M2 gouge might be compared to one made from M42 or some other steel?
The second reason is really a subset of the first. It is that some of these steels are harder to sharpen than the others. When I started turning, a lot of experienced turners used high-carbon steel tools for making their finishing cuts. The claim was that high-carbon steel tools could be sharpened to a finer edge than the (then) new M2 tools. Well, that wasn't true. M2 tools could be sharpened to just as fine an edge. What was true was that the methods these turners used to sharpen their high-carbon steel tools didn't work as well with the newer M2 tools. So, it wasn't that the M2 tools couldn't be sharpened, it was that the turners couldn't sharpen them. I think, but don't know, that something similar is going on today.
David Walser
Mesa, Arizona