Honestly I can't say if it has done anything. I have both a Cusco front and rear STB. I installed the rear properly by marking out two ovals in the panels and using a jigsaw to cut them out, then put a rubber grommet in place around it so it doesn't look so harsh. The panels don't sit the same way with the bar in place either because of the added height of the mounts so you usually end up forcing the screws in the hold it.
Like I said, I really don't think it's done anything towards the handing & performance of my car, I mostly got them for looks.
Our cars have a double wishbone suspension, if you had say a Honda civic, you'd most likely notice a big difference with a STB as they only have lower control arms (the older one's atleast) allowing the strut towers to flex alot more during turns.