Simon,
I had the same discussion on a thread some time ago, I too didn't really feel that Nismo's were worth it. They cost significantly more than standard, and after all, the standard ones do seem to hold up quite well. They are also just white metal bearings, that are supposed to be not contact with the oil film doing all the work.
However, I eventually decided it wasn't worth taking the risk, not with something that has high costs associated with it if it goes wrong, and went for Nismo.
Nismo mains also have a different oil channel to stock Nissan bearings.
Surely the multi layers are there for a reason. Most modern high performance bearings are multi layered. It is so that you get a hard support with a softer cushioned surface. If it was all one material, you would have go for a harder compound throughout, which may not be the right way to go, especially on RB's that are prone to spinning.