This is not meant to be argumentative, so please do not try to turn it into that.
Can gains be had by switching brake pads? Yes. Is it common or guaranteed? Absolutely not. At least not in cars that were designed properly. Most cars that have marginal braking systems are manufactured by the "bean counting" companies. BMW is not one of those companies. That is why they are more expensive than a lot of other cars. I have seen MANY tuner mags (all different brands) that have conducted big brake conversions on cars and tested them in a controlled enviroment with accurate equipment. Results? Not only did some not improve, but they were actually worse than the OEM! There have been some exceptions, but more so than not, they didn't offer any improvement, except for fade resistance, as FMINUS stated. For a track car, these are a necessity. Daily driver, hardly. Big brake kits offer increased fade resistance, great looks and a much lighter wallet. Most OEM braking systems provide more than enough clamping power to lock up the rotor/activate the ABS, at which point you are better off upgrading your tires for more grip to maximize the braking ability, also stated by FMINUS. This will also provide increased performance in other depts as well. The braking system on automobiles, just like the other systems (suspension, engine) is a balanced group of parts that are designed to work together. When you "upgrade" only a few components, you may see a slight improvement, but usually you don't or you cause more harm than good. That is the reason for bigger master cyl., proportioning valves, etc. so that you can match the system as a whole to perform to it's full potential. However, gains can be had. I've had Mustangs and Cobras over the years with upgraded braking systems. Results? The Mustangs benefited from the upgrades because they were very inadequate from Ford. The Cobras? Nope. Just better fade resistance, because they were designed with performance in mind, not how many pennies can we save. The OEM brakes on my 745? Amazing for a luxury sedan. They of course dusted like crazy and made my wheels black in two days. That goes hand in hand with performance, as I mentioned in an earlier post. Being that I didn't want to wash the wheels every two days, I tried some aftermarket "low dust" pads that were supposed to be equal or superior to the OEMs in performance. Mintex Red Box and Axxis Deluxe pads. Both were drastically superior in the dusting dept., but neither perform as well as the OEM. That's a trade off that I'm willing to make on a luxury car.
Take it with a grain of salt, just stating some facts, not opinion.