I also use headphones like I do with all my gaming unless my friends are around. Yes I agree you can hear a much more range of subtle sounds clearer than even a good surround sound system (IMO).
I am not sure what you mean about downshifts for turns, but I also have race music off.
I just got my tritton ax720's very good investment. Increase the entire game quality. Surround sound lets you hear when a car is gaining on you. IMO your in better tune with the car your driving. There are no outside noises to distract you.
As far as the downshifting i agree I recognize my gear and speed and upon hard braking you can hear the engine rpm letting you downshift without checking your gear... little stuff makes a huge difference