Are you sure the problem is mirage and not a resolution or contrast problem? Mirage, or more accurately "turbulence-induced blur", is a ground effect and the conditions are usually obvious: mostly flat terrain between the muzzle and target, and sunshine or thin overcast skies. Confirm the problem is turbulence by looking up at something high off the ground, like the top of a telephone pole. If the image is much clearer and has high contrast, then the problem is turbulence.
In that case, I wouldn't upgrade because the same image blur will happen with any scope of the same objective size. Aside from reducing the magnification, things you can do to minimize mirage include raising the gun and target further off the ground, reducing the objective aperture, and wearing yellow shooting glasses. Reducing the objective size from 50 mm to 40 mm can be done by cutting a 40 mm round hole in a black card and taping it to the objective.