So it's either one or the other.
Your preference is the OT 12 or DR's, .
It's not so much preference as using the right tool for the job.
remember I have all these 15" drivers lurking around and they do perform.
am positive they will do the job in the OT 15's
You have no idea how they will perform in any BFM design without T/S specs. Just because a driver is kick-ass in a direct radiator does not mean it will work well in a horn-loaded cabinet. There is a reason that there are so many different speakers - different cabinet designs require different driver specs.....and different drivers require different cabinet specs.
Bill designed these cabs and knows what specs are necessary for them to work well, he then finds drivers that meet those specs and puts 'em in the plans along with the specs so you can try to find others that work is the suggested ones aren't available.
It is a crap-shoot as to whether your drivers will work. They must have a rising mid-range if you are using the piezo arrays. They must not be too deep or they will not fit in the cab, etc.
There's way more to it than just the diameter of the driver.