When you place a woofer directly on a baffle the air displaced by the cone and outer surround radiates all directions it can, air likes the most direct route so interuptions are bad, if you put a woofer straight onto a baffle the air flows over the basket/bounting ring and onto the baffle.
If you recess the woofer into the enclosure or add an additional panel ontop so the outer surround is flush with the baffle you improve air flow from the woofer to the baffle, SPL cars use this to gain a few 10ths of a db.
If you look at your current mounting will have air pressure moving down into the void between your woofers, if you filled that void then the pressure can only move upward in unison into the rest of the vehicle cabin.
It is not much extra output but every little counts.
It is not worth changing now it is all trimmed and only a small gain but something to consider in the future.