The main thing, and I think Mlstrass tried to allude to this is that your spending A LOT of money on this for diminishing returns for what your getting. 2 12's 2 15's etc are only going to get so loud no matter how ****** the subwoofer.. 2 "nice" subs like your N3's or HDC4's something like that, lets say they do a 152. Even if you step up to a team sundown or some other mega priced subwoofer, your really only getting another 1000ish watts of continuous powerhandling and the cone area is identical. The main gain youd' see will be on burps that you really don't care much about.
At the end of the day, any kid with a wall and 4 $300 15's on a single 8k will likely be just as loud as you contiously, or even louder. The kind of power you plan on running to 2 subs is more than any sub is going to like. The gains will be minimal as your well into diminishing returns in every area.
At some point when you want "louder" cone area, extra coils and power ALL have to go hand in hand.. You can't just keep adding power and making good gains. Basically, I just don't want to see you get hella electrical, super expensive subwoofer and then blow the woofers trying to get more out them than is really possible. You have to reevaluate your goals and figure out what's actually worth it.
If you really want 2 15's in that car and want the loudest thing possible, you can go the way your planning. Just realize your spending 3x the money to get that last 5% of performance that will sound about the same to the ear. Your going to have enough money in the car to do a 160 and be stuck in the low 50's daily due to the platform choice. If the n3's aren't loud enough, no amount of power or new sub will fix it.
My next build in my SUV is going to be using some NSV3's.. Once I get used to that and want louder it WILL be walled. Simply due to that facts above. Sure I could go up to teams and get 1-2db louder, maybe 3 if I doubled up electrical again, but that's a lot more investment for litle gain. Walling it allows me 4x the box size, 2-3x the cone area and firing into 1/5 the original space.
We always say "it's all the install" as bassheads, part of the learning curve is figuring out what this really means.. In our case cheaper gear in a wall beats megabucks put into a trunk lol.