I would be willing to bet that a single Boston spg555 will be louder then a BTL 15 (SPL sub) yet cleaner then a Q 12 from them. Yes, the FI looks like a quality product and has decient specs but its just decient. The Fs is nice and fairly low but the system Q is too low. They will sound muddy on fast drum music. Even bass drums will be more of a loud thud then a sharp impact. No doubt the Boston will sound better then FIs Q series (their SQ sub).
It is an expensive sub but its the best sub out there short of a top end Focal but still louder then any Focal could do. I had a hard time selling them cause they were as much as a W7 or T1 and everyone was familiar with Rockford and JL but when I did sell one the customer was always blown away by them. If I was doing a deck upgrade on another customers car that I had already installed a couple W6 12's in and also just finished a spg555 install I would show them and they always say "Wow, wish I would have gotten that instead." 100% of the time.
IDK the OP was willing to spend that kind of coin. The spg555 would have been the only sub I would have reccomended.
I bet after he installs it he will post "Its almost too loud but damn it sounds good."
Another plus, the spg555 will never wear out. You burn it up and its only 6 screwes to replace the coil. You wear out the cone or surround and it bolts right in, no glue or mess.