I’m sure someone will get to this before I can get home and experiment with what you’re talking about, but I’ve had some difficulty in this before. There are some things to watch out for. For instance I found that if your object has thickness and you use the mirror modifier without getting rid of your faces on the edge of your model you will wind up with inner faces that can make for some nasty artifacts. Did you check clipping under the options? Something else to try if you haven’t already is the Mesh Lint addon. It’s a great help!