6. A dual vented enclosure with HP and LP is more like an 8th order LP and 8th order HP. This is a steeper roll off rate than the standard vented box...
Why do you suggest a dual vented BP design has a net result of 16th order BP, when in literature this design is described as 6th order BP ( for example X1, Rog Mogale, speakerplans.com ) consisting of 4th order HP and 2nd order LP.
Add 4th order protective HP and 2nd order LP electric filters and maybe out of band EQ and the design will end as a net 12th order BP.
Another example is the Eighteensound kit 18 ( dual 18" BP ), which is a 5th order BP design, similar to B2 from d&b or B10 from Kling&Freitag ( Access series )
In the net result there would be no difference to the direct radiating vented design regarding Group Delay.
But electric filters do not change in demanding applications