Get more sound of the box was to fire back into the trunk.
I find when doing the sub if you can rotate it then good. Generally, for me, more issue of what sounds better then what sound louder. I take clarity any day of volume. And to the point, only way you can know that is actual do install and rotate the box and listen insdie the car as what sounds best.
Ok if there isn't any time delay.. all speakers being equal (which they are in this case), there is a thing call the "Speed of Sound", (no this is not a bad coldplay song). The front speakers mite reach your ears in 3 milliseconds.. Rear might be 5 milliseconds.. and Sub could be 6 milliseconds. If you don't fix these time delays there is a loss in clarity. Most home stereo's these days have (Active Processor - Audyssey or cars MS-8) to fix this and many other things. But some car stereo at least have measures as how far the speakers are away from your ear to correct this to some degree.
Many times rear speakers also known as (rear fill) are removed partially because of this. That and rear speaker mess with front speakers sound stage.
2011 GT Coupe Premium | MT6 | 3.73's | Security Pkg | BREMBO | Comfort
Installed JLT oil catch can, Tint, Beltronics RX45/shifters