This is what my intention is
I'm with you on not being impressed with how well the silicon does with holding the styro together by itself, however I think the Drylok will help keep things together pretty well. I actually put one thin coat of Drylok on the back side of the BG to cover the pink and in my mind I think the silicon will adhere to that better than the styro.
I plan on using every bit of that 6 tubes on the back. I'm relying on strength in numbers for the background to stick.
Also, the physics of how it will work in my mind relate to the special snow/ice tires used up north in Alaska (instead of studs) it's called "siping" and it's basically a bunch of little cuts in the tread block that increase the number of rubber edges that contact the surface of the ice to give a car traction that it wouldn't otherwise have. When I look at the back of my BG and see all those edges of the boards that will be in contact with the glass and silicon, I reason that it will work using the same principals as the siping. (Look up Blizzak tires for a better idea)
Of course this is all just BS theory in my mind and I'm really just doing it based on blind faith in the internet community that did it before me and had only a FEW horror stories about not using enough silicon and having adhesion failure. I have yet to read anyone using massive amounts of silicon and have adhesion failure.