I just finished putting a sub in my Q50
https://www.caraudio.com/threads/2015-infiniti-q50.607606/
The problem is under the carpet in the trunk is a anti sway bar that goes across 25-30" from the back seats
So you lose about 6" of height which I believe maxed me out at 13"
I have 2 DD 2512's and 2 DD 3512's here and I opted to go with 1 DD 3510 because of space. Also because it was here already too.
Without knowing what kind of amp you're running it's hard to recommend a sub
I can tell you right now the stock batteries are ****, it's 150 amp alt so that's not too bad but big 3 for sure
If your doing SQ then a DSP over an LOC