a) No, RC does not allow comparison of edge holding between different steels
b) Maybe, RC does correlate to some extent with edge holding when comparing hardness of the same steel. However, if the steel is too hard (eg RC63), it may be very brittle and prone to chip out the edge. An RC of 59-60 will hold an edge better than an RC of 50 when comparing the same steel.
When your throw in non-iron based blade materials like talonite, all bets are off. Talonite normally has an RC in the low 40's, but holds an edge better than many much harder blade materials.
There is always a compromise of functional properties in the composition and making of blade steels. RC is only one measure of performance. A blade of 52100 at RC58 will not be identical in edge holding to a blade made of 440A at RC58.
RC measures resistance to Penetration. There are many other physical properties of steels that contribute not only to edge holding, but to other aspects of blade function as well (e.g. toughness and flexibility). RC is easy to measure, so its a commonly cited number. But its utility is limited and often overstated. There is a tendency of many production companies to push the hardness envelop too far. I personally think many blade steels make better knives at RC values slightly lower than sometimes produced. They sharpen more easily, and are tougher.
Size matters too. Small blades can tolerate greater hardness compared to large blade which needs to be tough for chopping. Differential heat treatment is probably the ideal way to make a blade because to edge can be produced with a very high RC, but the back of the blade and the tang can be made much softer, giving the blade both toughness, and great edge performance.
Paracelsus
[This message has been edited by Paracelsus (edited 11-09-2000).]