The existing SOI model is still good enough to give a more intuitive understanding of orbital mechanics. Also did they ever fix the Jool system in Principia? Never played with it but iirc there was an issue with Jool where one of the inner moons would very quickly be ejected from the system.
Given that it's a toy solar system, it's entirely possible that there's no stable configuration for Jool's system with its full set of moons :/