1-The "Mustafa" story line was played up for the movie. Chris mentioned once in his book about an Olympic marksman, but that sniper was believed to have been killed by other US snipers.
2-Chris did once kill an insurgent at a distance of 2100 meters. It was an insurgent with an RPG aimed at a convoy of troops.
3- There were rumors of an insurgent sniper named "Juba" whose reputation was well known during the time that Chris Kyle was being called the "Devil of Rimadi". Stories say that "Juba" had over 120 kills of America servicemen, although their is some who believe these kills were done by a number of snipers, not just one.