I too find L2 and R2 to be better than the stick for gas and brake. Not only do you (IMO) have finer control, *but you can switch between them instantly *with that same amount of control, and can even use both at the same time when desired(for example, to help free locked-up wheels while still decelerating or to blip the throttle to help with downshifting). Of course, I'm no expert anymore with a DS3, but it's not gas and brake I have a problem with after using a wheel now for years, it's steering.
If you only ever use the brake as an on-off switch, you are missing out on half of the fun of high-performance driving, even with ABS on.
Oh and I find using X and Square(or Square and O, or whatever works) for the gearshifting is easier than L1/R1 when using the triggers.