First of all, if I can backtrack a little, the issue of police investigating police is perhaps an oversimplification. As I think you have correctly outlined, it applies to certain serious issues. Amongst those, I would include serious bodily harm, deaths while in custody, and issues of sexual assault.
In those instances, and perhaps in some other areas, I believe it is appropriate for a body outside of the RCMP to conduct the investigation. I think it's too difficult for the RCMP to maintain its credibility with the public when these issues come into play.