Would an area-ruled fuselage improve the MiG-15/17's performance?
In a word, yes. But that sort of depends on the definition of "performance". The top speed would likely be effected only in a minor fashion, but the transonic acceleration would likely improve, as would the fuel economy.

It is worth pointing out that, largely by luck, many of these aircraft got some of the benefits of the area rule without specifically designing it. The location of the cockpit behind the intake required the fuselage to become larger in that area, especially when one considers the canopy, which ends just in front of the wing. And then the tail begins near the end of the wing and the fuselage begins to narrow behind the engine as well.