You also might try some aileron differential. High wing, cambered airfoil airplanes have a tendency to yaw against the ailerons. By introducing differential movement of those ailerons, the extra drag on one side that causes the adverse yaw is cleaned up, and the plane doesn't fight the roll.

I have a similar model that had your problem. It wasn't a Sig but probably came out of the same Chinese factory. From the first differential added, it started responding lots better. I think it got about 2 or 3 increases over time. The changes were done to the rigging so were done in steps.