You will definitely lose mpg's if you start changing things. If you pop off the steel panel in the trunk that the batteries are behind, you will see there is some space in there but I wouldn't advise putting anything in there. You will notice a vent in the rear deck and I think that is for ventilation. If you are really wanting to bring bass into the cabin, make a port up through the rear deck.
I would also advise putting some dynamat down. The tires are very noisy and you really hear the road noise in the cabin. IF, you learn how to drive it, you might get closer to 50mpg's and if you learn to hypermill (i don't) you might get into the 60's for mpg's. But, then again, if you are putting a system in the car, I highly doubt hypermilling will be your bag of tea.