Will adding a Rear strut tower brace to a RS upset the factory suspension tuning?

will a sway bar or something esle have to be changed?

 

It should "enhance" the stock handling, rather than drastically improve them. The Evo is damn good out of the box, but numbed down a bit for daily driving and all out performance. You will feel a bit more difference with a swaybar than a rear strut bar. I have friends with the Cusco RSB and they can't tell the difference. I lowered the car with Hotchkis springs and got an alignment and that made a good difference vs. stock.