First of all the stacked betsy shape would probably be really nice giving the ZOB more midrange weight and more powerful projection.

Before I would put a similarly low Q driver into a ZOB design, I would rather hear it with a higher Q driver.  Having to increase the size of the ZOB in any way starts to bring in lots of competing designs, like the Big Betsy which would just smash it into the dirt with a similar amount of real-estate.  Putting a Betsy driver in a ZOB or similarly higher Q driver would have more bass and the ZOB cabinet size would not have to change.

On the same wavelength, scaling a ZOB way up to accommodate the higher Qts  F15 driver would likely edge out the Big Betsy Baffle by extending the bass another decade at least and the texture of that cavity properly stuffed is really a positive enhancement to the midrange, not just low bass.

Steve