I really think there is something in what fried said. For one thing, it is certain that when the head chef is not there, the stand-in chef certainly uses the same recipe, but the flavour difference is obvious. I've experienced this many times in many different favourite restaurants through the years. Much of the final flavour is clearly in the method.
Also, on a personal level, I switched to the BIR base sauce approach about a year ago, and in that time I have found greater flavour improvement from changing the method as opposed to changing the ingredients. In short on that point, improvements came from frying garlic & ginger in both the base and final curry, frying all spices (other than garam masala), cooking the final curry hot and quick and to the right consistency. That last point took me a while to get right; no book can teach me that, only experience.
Another big leap towards BIR came from the right use of whole spices at the right time, such as brown cardomom in saag and star anise in jalfrezi. Small things, big difference.