I think that you are missing one thing, by putting subs in the rear deck you still have to separate the back wave from the front of the driver. speakers work by creating pressure differences that correspond to the AC signal that is music. if the front is not sealed from the back, you are not creating pressure differences, your just moving air, and that will sound bad, very very quiet, and bad. That means that you can either build boxes for the drivers under the deck and seal it to the deck with fiberglass, or you can seal the trunk from the cabin, that means the big hole behind the seats and the various little holes in the deck itself.
To complicate things a bit more subwoofers are also designed to use the increase in air pressure inside of a box to help brake the cone, so it doesn't travel too far and either slam into the t-pole or rip the spider. Their are drivers that have suspension systems built to handle being outside of a box, they are called infinite baffle drivers, and they are not anywhere near a common as normal subs are, you can find them, but with 8 inches as your goal your options will be drastically reduced.
Ohh, but proper infinate baffle setups are teh sex, they sound really really awesome, but not that loud.
EDIT: If you plan to go that route this would probably be your best option.
JL Audio ZR800
A sealed enclosure would probably be the easiest way to get the sound that you are looking for, a single 10" or twelve inch in the trunk with 500W would be enough. You could go ported with good results too, but it would take a good box design to do that and still get the sound that you are looking for.