In the case of focussport, their front stress bar = a strut tower brace. Their rear stress bar is a rear strut tower brace. The idea is to help eliminate some flex in the chassis. While I can't really argue that there's no benefit to this, I personally consider them to be a waste of money, unless your tires are so sticky that you can actually benefit from reducing a few degrees of tire deflection caused by the unibody flexxing. However, it's a cheap mod that can't really hurt anything (besides adding a little weight), so if you like the looks and you feel that your engine bay is too twisty, go for it.
It's hard to see what you actually NEED, though. Assuming you upgraded the rear sway bar to help solve a problem of understeer, then you need to get some seat time and see if that solved your problem. If your car oversteers now, then you may want to consider a front sway bar as well to balance it out. A lot of it comes down to personal preference, and getting the car to do what you'd like it to do. I wouldn't by any of that stuff (stress bars or sway bars) unless you have a specific problem that you're trying to find a solution for.