So discovering this may have put me out of a project I was going to do this summer. I was planning on modding the stock physics to replace the patched conics approximation currently used for determining the orbital paths of spacecraft with a matched asymptotics approximation, but you seem to have beaten me to the physics improvements (as I assume you are completely replacing the patched conics with N-body movement). That said, I'm not familiar enough with the KSP code to know exactly what you are changing, so I'm uncertain if the approximation I am proposing would still be useful as a mod. Still, if you could point me to any specifics about the exact code KSP uses for it's modeling, I would be very appreciative (I know it uses patched conics for the approximations and the planets are on rails, but not much beyond that).