Jeff,
15N20 is good stuff, yes it has plenty of carbon, somewhere on the .70 range.
15N20 and 1084 makes a great blade. Great. They will average out in the high .70s (.070%)
The problem with putting, and I do mean putting, nickle in is that pure nickle wont harden and it cant help but have soft spots. People do it, but I dont care for it much. Lin