We offer quite a variety of them and have tested all of them...
What you will find is the trade off is power handling and xmax...power handling goes down as the motor force decreases heavily with shallow mounts. Their magnets are tiny compared to a traditional sub. As well as the mechanical soft parts of the sub - the spiders, the surrounds, etc the motor also dictates the control of the cone. You dont get massive amounts of control as the magnets cant create enough magnetic field to tightly govern its movement.
The xmax of the sub (the amount it moves back and fourth), is also decreased due to lack of control and lack of physical room for the voice coil to move up and down alot.
That being said though, will you notice the difference between most subs...not really! Well, not unless you listen to a huge variety of subs day in day out!
We have done an install with 4 12" Ground Zero slim subs and it sounded great especially as it was a shallow build!
Your best sticking off with proper shallow mounts like Gladen Audio, Ground Zero etc They are leaps and bounds ahead of Pioneer, Alpine etc for shallow mount subs!
You also dont have to go shallow mount if you get creative...
Heres the build in our Corsa C...has 2 12s in the boot and is NOT higher than 3inches from the boot floor The reason it looks like we used alot of room is we have inverted the subs so the magnets stick out