I have never adjusted either of those. This is my perception of them, but not 100% certain. Leaving MixThreadCount at 0 lets SONAR determine the core count and use them appropriately. EnableSetThreadIdealProcessor lets SONAR choose which thread to load tasks to (if allowed).
 
That said, both of the above leave SONAR free to determine situation as it can, and SONAR does a pretty good job of this. Also bear in mind that SONAR is a host, so a VST that is specifically scripted for Core 0 will go there (SONAR cannot override its coding)... in situations where Core 0 goes through the roof, take note of what VSTs you have inserted (Ozone used to be one of the common offenders for this, but I am still on Ozone 5 so not sure if still true).