1 18" has about the same cone area as 2 15's? i dont think so. especially with the the kickers being square. cone are of one 18" = 254.47. cone are of 2 15" = 353.43. and that's a round sub, so kickers would have more cone area being square
so imo...the BTL definitely has potential to get much louder, but given that you're going to have the same power, and drop in cone area, that it wont be louder. but then again the type of enclosure also comes into play. 80% enclosure, 20% build