Great test. My Stretch CF was 64.5 RC, and my M4 Mule was 63, so it seems those Spydercos were softer than the norm. I'll report back in a week or so on the Hardness of my 2 M4 Millies, my Gayle Bradley, and a few others like my second S90V Manix 2 (my first one came back at 58.5-59 RC). Either way I'm really impressed with M390, if it is there with M4 and is stainless to boot it sounds like a really nice steel. It also appears to take a very nice edge, if it is anywhere near M4 in sharpness it appears to be a really nice steel. I think Carpenter 20CP is similar to M390, and I am pre ordered for the Para 2 in that steel so I should get a good look at how it performs (if I don't get a Phil Wilson custom in M390 first).
I have a Benchmade Rift in 154CM that is 61 RC, it would be interesting to see how 154CM compares to that S30V at 59.5 in your tests. I know my Rift performs really good for 154CM, taking a nice edge and holding it nicely, but ZDP, M4, and S90V have been in my pockets a lot so anything in the 154CM/VG-10/S30V class seems lacking to me in edge retention, but it is all relative. If you want to borrow some of my folders in M4 that are harder than 62, ZDP-189 at 64.5, or S90V at 59 or so I'd be glad to ship them off to you in a couple weeks after I get them back from testing. It would be interesting to see how much a couple extra points of hardness effect your tests, and S90V would be new to your testing.