The soak temperature is determined by the desired core hardness, since the soak temperature never gets high enough for the sides they never harden and hence the blades turn out very soft as a whole. The performance tends to simply be determined by how hard the core is and how thick it is, how much of the knife is actually hard.
Most of the Japanese laminates, which use low carbon stainless (<0.15% like 420, though usually 410 since it won't harden anyway, some times they just use wrought iron or really low carbon almost mild steel) are very thin cores at very high RC levels and thus stay sharp long but are very brittle at the edge and very weak throughout the body. 420, which only gets to a max hardness of <400 Vickers at its right soak ~1850F simply doesn't get hardened at the much lower soak for carbon steels.
Using a high carbon stainless like 420HC which is a very different steel than 420, or 410, would be of little benefit because it also would not get hardened, even though it can because again the soak temperature would never get high enough to get decent hardness, 420HC maxes out at ~55 HRC (depends on the grade the carbon percentage varies a lot) or even 440C if you used it would make little difference, that would just be a waste though which is why very low carbon steels are used since it never gets hardened anyway.
This is why laminates that use higher carbon stainless for the sides tend to use high carbon stainless steels at the core as well (like a 440A and 440C, or 420HC/VG-10) and again they do the same thing, harden with the specifics to get the core where you want it, and leave the sides much softer.
-Cliff
