IMO, the difference between 42% protein and 52% protein isn't going to have much (if any) influence on weight in a normal or underweight dog. I would eventually switch to the regular Evo.
What makes the Evo RF special is that while it has an ultra-high protein content, the fat content is moderate, which is more conductive to weight loss in obese dogs than your run of the mill carb-stuffed "lite" or "diet" foods.