Once again I don't know if this helps but..
I would get the capacitor if I got the 10 or 12. It's not that I doubt that the battery is strong enough for it, but the "W7" series are probably the most powerful subs on the market and according to every so called "pro" I talked with they are under-rated. So when I get mine I'm just going to spend the extra $100 or so and get the cap.
I'm not very optimistic, so when it comes to my cash I always have a better safe than sorry attitude. That's what brought me to the Bimmer in the first place!
Me and my heart