Some bit of old timer's tricks and lots of snake oil salesman.
You'll find that BFM subs are not well respected here, or really, anywhere outside of the BFM forum. There are no real measurements, and there is no one using them for real shows.
The V-plate thing is old. Really old. Folks in the way-back were doing this because and extra couple of dB of efficiency was worth it. No one does it now because power is cheap and new (within the last 30 years) drivers are worlds better at reproducing very low frequencies. We just don't need to carry around plywood and stack the subs in fancy groups to get them to work. Frankly it's a bit insulting to the client to suggest you need to do this in front of a stage.
I doubt the angling-down thing does much of anything outdoors. You aren't appreciably changing the mouth of the horn since there are no sides. If you added sides, it might do something, but what?!?
The corner-loading thing is true. But it's true for all speakers, not just BFM subs. There's no magic there. If you look at manufacturers spec sheets they will (or should) say how they were measured. 1pi, 2pi, 8pi, whatever. Pi in this sense is marketing wank to say it's in free-air (1pi) or on the ground (2pi) or in a corner (8pi), etc. You'll likely only see 1 or 2 pi being used by a decent manufacturer. 1pi being the best way to measure a frequency response, but it's the hardest to make the speaker look "loud" since there is no boundary gain.
BTW, Boundary gain giving you 6dB of boost is the theoretical best case scenario. That's probably like a concrete floor. If you put it against a studwall you won't see as much gain because the wall will flex a certain amount.