Upgrade brakes really are worth considering
A thought to throw into the mix on this. Whether or not you go for the M Sport brakes is less about perceived power and more about feel and control.
I've a bit of experience of upgrading brakes and, once you've done it, it's a revelation and you will never (never, ever) go back to basic brakes. A bigger braking system with bigger, better vented discs and bigger callipers will give you a firmer pedal feel that is much easier to modulate (sorry, can't think of a smaller word to say this). The bite will be really positive, with a great sense of feel and a really good sense of exactly how much further you can go.
You may not find fault with standard brakes in day to day driving, but the reduction in stopping distance with bigger brakes, especially at motorway speeds is very significant. The thing that finally tipped me into buying serious brakes (for my 530d M Sport) was having a very near miss when a woman in the outside lane on the M5 north of Bristol managed to spin her car a full 180 degrees in front of me at speed (still not sure quit how the heck she did it, as all three lanes were busy at the time). I always leave a healthy gap, but I lost half a second as my jaw dropped at the scene unfolding in front of me in slow motion - then I hit the brakes hard and stayed on them. I was doing an indicated 85 when she started to spin and she was stationary and (miraculously) facing me, perfectly lined up in my lane seconds later while I was still moving forward and the pedal was starting to sink ... I just managed to stop and actually nudged her car ever so gently. By the way, the car was 18 months old and in good shape - it's just a rare thing to do a full-pedal stop from 85 to zero and 1850kg is a lot of car to haul down full-pedal from 85 to zero.
It was all very scary: first the near miss (erm very small hit actually) and then the horrifying sight of three lanes of hard-charging traffic bearing down on me from behind and taking horrifying aeons to realise that I was stationary and married to the nose of an MPV pointing the wrong way. I really thought it was a matter of seconds before I'd get hit very hard and wound up in an ambulance, but luck was with us I managed to get across three lanes to the hard shoulder. Too much heat generated too quickly for most standard braking systems to cope on a heavy car.
Shortly after that, I took the plunge and put a set of big six-pot AP Racing brakes on on the 530d and have loved them ever since. You do have to be careful in a motorway situation, as you can stop in less than two thirds of the distance of most vehicles on the road. You'll see less difference in stopping distances at lower speeds (where heat build up is barely relevant), but the added bonus in all situations is a much firmer pedal with loads of feel and feedback.
It's worth trying a little test with standard brakes to get a sense of what this is about. By the way, ONLY DO THIS if your dampers are in good condition, or you'll wind up swerving across the road. Try braking two-thirds of full-pedal if you're not sure. Find a wide, long, straight stretch of road and go there when there is no traffic. Try three successive full-pedal stops from 85mph. As soon as you've come to a stop, get straight back up to speed and hit the brakes again (stay hard on them all the way to standstill as if in a real emergency stop). Chances are, you'll feel some fade and a bit of pedal sag on the second stop. By the third stop, you'll probably be feeling real signs of fade.
Anyway, I know the mildly upgraded M Sport brakes won't be in the league of the likes of AP Racing kits, but the main thing I'm aiming for is firmer feel, shorter pedal travel and better feel and control. I also want the aded safety margin - bigger, better brakes will simply haul a car to a stop quicker and with less drama in an emergency braking situation.
Although not up to M Performance spec, the disks on the M Sport brakes are quite a bit bigger than standard, more heavily vented and the callipers are bigger, stronger four-pot units. It's worth noting that BMW fit them as standard to all their hottest non-M cars (e.g. M135i), so they can't be bad.
If I had the money, I'd go for the full M Performance brakes, but they're way more money than the M Sport brakes and I think there will be a very noticeable difference over standard - not bad for £560.