About as competitive as Serena Williams vs the 203rd ranked male pro-tennis player (the guy won 6-2 just after having a beer).
Look, this is silly. The Canadian national women's hockey team, 4 times Olympic champions, play practice games against 15-17 year old local boys hockey teams and lose as often as they win (and the boys aren't allowed to body check). The Swedish woman's soccer team, Olympic silver medalists, lose in practice games against 16 year old boys. The fastest woman's 100 m sprint time, a steroid induced time that no woman has come close to since, is regularly beaten by high school boys.
The best women in the world, in sports far more competitive than MMA, lose to local high school boys. And you want to put a woman against the top men in her sport? It'd be ugly, one sided, and horrible for both Cyborg and WMMA.
Could Cyborg beat the average untrained male? Easily, and their weight wouldn't matter. Could she beat 95% of Sherdog? Again, easily. Could she beat a professional male fighter of any weight? Not even close. This is like asking if the best WNBA player could outplay the worst NBA player - its just not happening.