I can tell you one thing, though. It may be the flywheel. See, transmissions make noise (especially the flywheel) as they work, it's no big deal. But this noise scares a lot of people, so what manufacturers do is, they add weight to the flywheel and make it bulkier so that it pretty much "deadens" the noises that come from the transmission...that's why if you've ever looked around for aftermarket flywheels for your car, it is easy to find an aftermarket flywheel that weighs like half as much as your stock flywheel. It saves weight and improves engine response, but when engaging the gears, you will hear transmission noises and most people are not comfortable with that, even though they are completely normal.
My point is that perhaps BMW wanted to use a lighter flywheel on the 330i Performance Package, so they sacrificed a bit of the sound-deadening materials for less weight and quicker engine response....just a thought.