I'll try and offer some help here, the larger the driver, 8" - 10" - 12" the more cone surface area, = more Bass... Going with a smaller driver 8" vs 10" doesn't mean less accuracy or poorer bass response, it just means less output.

 

Will two JL Audi w7 12's fit in the trunk of the 350z. With or without a custom box? Thanks

 

They'll most likely fit with a custom box, but you'll end up using all of your hatch space...

 

While this is "technically" true, it disregards the fact that a larger surface area requires greater force to move the same distance. IE you can move an 8 easier than a 12. Seems pretty obvious, but in fact its not even the biggest concern.

The biggest concern is "turnaround" - basically how long it takes for the driver to stop the cone and turn it around pointing the other direction. Figure an 8" sub has to turn around 50.24" of surface area. A 10" sub has a surface area of 78.5". A 12", 113.04"... (area = 3.14(radius)^2)... (note this assumes a "flat speaker, and is not truly accurate until you had the curve of the plane as well, but it gets the point across)...

So, the difference between an 8" ---> 10" : 28.26" of surface area. Meaning greater output, but conversely more effort required by the driver (driven by amp) to move the speaker.

What does this all mean? A high-output amp will perform better regardless of what you put on it, but an 8 will generally sound better with a poorer amp compared to a 10 or 12.