Not much louder. I garuntee the sub would blow. If you wanna get the most SPL out of your sub put it in a ported box. Tune it to 35 hz for SPL and SQ. if you wanna make a pure SPL box then find the resononce frequency of your vehicles cabin. And then tune your box to that frequency.
After pushing a sub to it's mechanical limit(just one 1000/1 is capable of this) you can't get any more SPL out of a sub. After pushing the sub to it's mechanical limit, the only other way of getting more SPL is through cabin gain, you can get higher SPL by tuning the box to the cabin's resonant frequency as Wahl said. Other methods are simple ones such as coupling the airspace between the trunk and cabin (if this applies) angling/positioning of the box, and experimenting with different boxes.