i only just replaced my o2 sensor and only noticed a slight difference on c34 series 2, im assuming computer is relearning now but was getting rubbish before 300l sometimes less to a tank but its moving its way up to 330l and thats with me driving it lately not the wife so heavier foot lol, freeway managed 550l to a tank.
This is with nistune, ss2 turbo, fmic, cat back zaust, z32 and stock the rest