FWIW, I got my best numbers at my lowest tune and the best experience from a sub stage when I did subs and port forward and sealing the trunk off. I will never fire into a trunk again. The only real way to know what will meet your goals is to build and test. I gained 3+db even after dropping my tuning to 28Hz after forward firing and sealing the trunk off.
He posted in on Caco the other day. Of course, its a general rule, and not every single car will follow it.
Im going with a trunk wall with 2 15's. Hopefully its efficient!