Actually, we will have similar output. We tested a dual opposed 18" dayton sub (the dayton drivers are tested to be almost exactly like mine) and the output was just a touch more than my Submersive. 2 Submersives have the same output as 1 Cap Sealed 2. So, it would take 4 of my drivers to have a bit more output than 1 of your S2's.
So, 8 SI drivers equals a bit more than 2 Cap S2's.
The reason why I went this route was the price. I paid 1400 dollars shipped for all 8 drivers. If I could have afforded it I would have just bought 2 more Submersives and had quads, instead I'm selling the Submersives to fund this project. I already sold one, and if I like what I hear with the first couple SI subs I'll sell the other Submersive.