Sway bars and shock tower bars are two different things for two different purposes. The sway bar stiffens the rear suspension, reducing body roll and changing how the car handles. The result is less understeer and more front grip.I'm with everyone here. A RSB is more bang for the buck. Chassis bracing does work if you have a lot of it but with just a single bar it won't make a noticeable difference at all (if any).
Besides most STB are for looks anyways.
The front tower brace keeps the suspension parts in proper alignment by securing the upper mounts together at a fixed distance. When the upper mounts are not tied together, they can move independently, especially if the car has Macpherson struts. When this happens the tires can lose grip from camber changes. Just think of it as a rigid box with one side missing. Without the top, the two vertical sides move independently and can flop around. Add the top section and it is a much stronger structure, not nearly as prone to move around. Some cars benefit from this, some not so much. The 3 seems to be one of the cars that does.