Yield on a Boneless Turkey
Chicken breast, not overcooked, loses 10%-18% of it's weight during cooking.
Turkey will be similar.
 

As you can see, brining a bird, either wet or dry, results in less moisture loss.

Your 15% solution undoubtedly contains some salt but I doubt it is enough to be considered fully brined.

I would therefore, expect your yield to be around 80%-83%. Obviously, if you overcook the meat you will lose even more water.

Of course, these charts were made to demonstrate the effects of brining and not as an exhaustive study into poultry yields. For 700 people you would want to get this as close as you can. Personally, I would salt and cook one of them today, weighing before and after to give myself more confidence.