Hi liquidsilver,
The volume behind an in-celing speaker is essentially infinite if it extends into the roof space.A back box as you propose would be anywhere from 10 litres to 20 litres ; the same size as an average bookshelf speaker
An in-ceiling or inwall speaker may actually benefit by being enclosed in a smaller backbox as you propose. However the increase in bass response is in a frequency range where EVERY inwall/in-ceiling speaker is ALREADY boosted simply of it's location: being loaded i.e. assisted by the wall/ceiling surface. ALL speakers (inwall / in ceiling /bookshelf / floorstanders / whatever ) receive a mid bass boost 100-300 Hz simply by positioning them close to (inside ) a wall or ceiling boundary! This is the reason why some custom speakers can sound chesty and congested. That isn't BASS! BASS is below 100Hz
The best inwall speakers can be virtually guaranteed excellent performance if equipped with a back box but since custom speakers (inwall+in-ceiling) are just that "custom" put them where you want or where you can. This unknown entity makes the speaker manufacturer optimize the speaker for the largest volume behind the speaker. Every house is different and every location will vary as well. The left speaker may be in an interior wall where the right speaker may be in an exterior wall with fibreglass and brick behind it. I think you get the idea. . .
The best reason for using a backbox is to reduce the sound transmission from room to room. There are many electrical codes and bylaws that actually force you to use a metal backbox for speakers in many commercial and industrial installations.
If you are going to go to the trouble of building the backbox, surpass whatever any manufacturer can do and build it better. I assume you will be working from above the ceiling.
Build the box with 1/2" MDF with a internal volume between 10-15 litres.
Staple a piece of fibreglass to the back panel to act as damping ( will reduce sound bouncing within the box)
Use silicone sealant to completely seal / adhere the box to the drywall. Seal the hole where the wires enter the box with sealant as well.
Wishing you success with your project.