I've uploaded a video showing how the sim behaves if the CG is moved five feet forward. It makes a drastic change, and the plane behaves much more as Concours77 was expecting. It is not how N137US behaved, of course.
The precise numbers are probably not important, since there's so much uncertainty in the sim in the first place. It's an approximation of the plane behavior, and I've drastically altered the plane to simulate the damage. So the sim's value is in showing broad trends and ideas.
As I say in the video, if this behavior is the explanation behind how N137US got to the final wreckage site, then N137US behaved more like a sim with the 20 foot CG displacement than 15, regardless that the real life displacement was some different value.