In comparison, my sol is a little louder than my buddy's GSR. I do have more aftermarket parts that increase the sound volume (headers, exhaust), but it's not too bad. It's way better than my buddy's CX hatch. That thing has jack for sound deadening from the factory, so it's loud as fawk.
The flexing chassis is GREATLy diminished when you get good shocks and some strut bars on the car. The shocks being the biggest factor. Even with tokico blues, I hated the flex. Now with KYB AGX's, it's WAY better. My Si sits at about 4500 going 80 in 5th. I still always get 32mpg when I beat on it out on the freeway. If you take it easy, I can get about 35....