I don't know if it would fit in a 3 series due to (from what I understand) a bit less clearance under the seat. I would basically have to make a spacer for most options so to go smaller I would just have to make that spacer an adapter ring. It was recommended by CDT due to it performing reasonably well in a lot of different enclosures, it has decent excursion, and it can handle the power from my amp. That and at $200 a pair it is a pretty attractive price point.
Also that BMW set is 1", 3", 6.5" not exactly ideal. But they do have a lot of 3 way crossover options. My plan is to run the component sets down to probably 120-150Hz depending on how they perform in the car, and these little subs from 50-150Hz again adjusted by ear to tune them.
I have CDT speakers in my wife's car, and I have really enjoyed them, so that is why I am going CDT for this system as well. Plus the 6 1/2" woofers in the doors of my wife's car put out a good amount of bass, so I figure that a 6.8" sub woofer should be a bit better. I will also have the option of getting 3 way crossovers in the future and adding a sub in the trunk if I find that these don't handle the bass as good as I am hoping.