SFGs, like additional fuselage side area (because that's all they really are, except for their additional function of inhibiting spanwise flow, which has little or no effect on lateral stability or maneuvering by itself), have an unknown effect on the ability to follow a given ground track, unless more is known about the SFGs.

And here are the simplified, non-engineering qualifications: If the AC of the SFGs is in line with the airplane's CG, there will be no effect on lateral stability, though maximum lateral lift will be increased. If the AC of the SFGs is aft of the airplane's CG, they will increase yaw stability and partially reduce the amount of extra lateral lift available. Presumably this is self-defeating unless the additional stability is desired for some reason. If the AC of the SFGs is forward of the airplane's CG, they will decrease yaw stability, which may or may not be desirable, depending on the degree of yaw stability shown by the airplane before installation of SFGs, and pilot preference.

One cannot simply say SFGs increase or decrease lateral stability without considering their geometry and placement, any more than you can say that increasing fuselage side area increases or decreases lateral stability without specifying how and where the additional area is distributed.

It is well known in aerodynamics (and intuitive) that cowlings and other lateral area ahead of the CG are destabilizing in yaw, and area added aft of the CG is stabilizing.

<reduced troll-factor>