1) I agree with the weight placement however you're not thinking about the whole picture.
a) Relocating the battery to the trunk will only add 6 lbs (wiring & battery box) similar to BMW's weight method
b) Installing a light weight battery in the stock location will yield few results and more issues down the road
c) Read this for more info about light weight battery choices if you must ->
http://forums.nasioc.com/forums/showthr ... ?t=1852892
"..From the research I have done, you are going to be safe with an Optima yellow top. If you want to get extreme, some of the lightweight batteries like Oddesy, Deka, Braille, etc are great. They tend to have more problems with long term reliability, complete discharge recovery, and cold starts. Take this into consideration. Especially if your in a colder climate..." -roninsoldier83
3) My generalization sums up the basics of diaphragm response/movement and the general genres that respond better/worse to them. Not to knock your setup but there is a reason why the 13.5" woofers are under priced and can be driven harder. If your looking for the lower frequency response's then you have the proper setup, I am just point out that they will not run efficiently compared to smaller woofers. This is the reason why the pros use multiple sizes instead of just **" twins. Its your setup do what you want just take this with a grain of salt..
Cheers & good luck with your voltage issues.. I am off to finish the T-shirts..
-Chuck