Thats a fair question, but I disagree with sam's assessment that their is no DD benefit. I only learned about this swap this morning, so based on what I have been able to ascertain, I have two schools of thought on it.
First, I want to clarify that I only plan on doing this to the front. I need to get new pads and rotors anyways, so I am only paying for new carriers and calipers. I can buy a set from NAPA (for example) for 200 for the pair, so this is only costing me a small amount of money.
Now as to the reasoning:
1. why did BMW put bigger brakes on the 330's? The car is not significantly 'sportier' (not in an xi trim anyways). Its not significantly heavier either. The only reason I can see is because it represents an 'upgrade'. Bigger is better in brakes, that shoudnt be in dispute. What is in dispute is if the 325 brakes are inadequate (they are not), but you could argue that about any number of modifications that people make to their vehicles (whats wrong with the stock clutch, oem tires, oem shifter, etc....)
that brings me to my reason:
2. I do mostly stop and go driving with the occasional rip out in the countryside. I had pretty bad brake fade the last time I did this, which is why I am doing new pads/rotors (regardless). I also am pretty hard on my brakes, when I learned to drive stick, I didnt really cotton to downshifting into stops, so I use my brakes quite a bit to stop the car, so again bigger is always better.
And my last reason, because I am pretty much dead set on turning my base level xi into a car its not. Upgraded to style 44's, gauge cluster rings, lip spoiler, etc... maybe it will get mistaken for a zhp some day