The loudest box ive had so far was about 2.6-2.75...(i dont remember exactly)...after port and sub displacement. I think i had it tuned to about 35hz. I woulda went a lil bigger if I could but i barely got that box in my trunk as it was. It sounded good. How much power are you putting to your SSI?
Try aeroports if you can. Maybe a 3" or 4" and it should be awesome. I've heard an aeroports vs. slots, and i have to say i like teh Aeros alot better. Next box i build will hopefully have 2 4".