I would stay away from HSS steel for a number of reasons. From the little I have read about HSS it's not worth the trouble to try and make a blade from it. High speed steel is alloyed to stay hard when hot hence the "high speed" part. Drill bits are HSS and the way they are heat treated they are barely hard enough to make a good blade but they stay hard at a higher temp then normal "boring" steel. Another steel you mentioned was a "very high carbon steel" when it comes to knife making on a realistic level anything over 0.6% carbon is getting high enough. I would say in the knife making world 1095 would be your "very high carbon steel" and it won't give you a much better blade then 1084 or 1075. There's a point that carbon content doesn't matter any more as far as blade performance goes. A properly heat treated 1084, 1095, 5160 blade (any lots more) will out perform almost any of the "super steels" as far as toughness and cutting go. The only things that the other steels have going for them are a little better abrasion resistance and of course the corrosion factor. Some of the "best" Japanese blades out there are simple "high carbon steel"
I have been a knife but since I was a child and I always wanted the "best blades" I could get. I've owned a lot of knives from the cheapest to some very pricy blades. One thing to think about is this. A company like say spyderco has to warranty their stuff. The steels they use are very good BUT they are simply not the best because of the warranty factor. They are not going to sell a product designed to have the ultimate in edge retention because it would be brittle.
Make some "boring" carbon steel blades and heat treat them properly you will be amazed. A well made hand made knife will (in my books) always perform very well.
Why do you want a rc68 hardness?