If im repeating anything anyones said im sorry, havnt read the whole thing..
I allways left foot brake in games and on the road (in my automatic). I allways keep my left foot hovering just above the brake pedal whether its in game or on the road. That couple tenths of a second it takes you to move your right foot back up 5 inches off the throttle and over to the left 3 inches will make a significant difference over the distance of several laps and on the real life roads that tenth of a second can mean getting in a wreck or not, as well as in the game actually.
Just try tapping both your feet (as fast as you can) as if you were alternating putting full pressure on a gas pedal, letting off, and then putting full pressure on the brake with the other foot and go back and forth. Then try (imagining) putting full press. on the accel and then brake alternating using only one foot, its physically impossible to do it faster than using two feet unless your coordination is just horribly poor. Even if, amazingly, you think it is possible, you still must admit you dont have as much immediate fine control because your having to use a whole lot more muscles to get your whole leg to move instead of just 'tapping' your feet on the pedals by just rotating your ankles a bit and using a bit of your two calve muscles.
Why have one leg sitting there doing nothing when it could quicken the inputs by sharing the work load? Its just a matter of practising for a bit with both feet. Took me about 4 days in my car before it was second nature.