In theory, anytime you double the power or double the cone area, you will get 3 dbs more.
Myself, I like to keep the bass/component ratio around 2-1, so if you have say 800 watts to the components, I'd run 1,600 watts to the subs.
Having said that, I think you could put a couple high-end 12's in there (as long as they work in the current boxes) that will easily outrun those 8 components.