The HCCA are extremely hard hitting subs if you have the juice to run them correctly. They are very inefficient (87 dB 1w/1m) and need tons of current (3000+) to reach full potential.
But, of course the 18" BTL's will generate higher SPL on the same amp. Not only because of the substantial increase in surface area, but because of their much higher efficiency (93dB 1w/1m). Technically, the HCCA would require 4000RMS to equal the output of a BTL given 1000RMS. Now if we're talking the 15" BTL vs. the 15" HCCA, both fed the same (efficiency adjusted) power, it would be very close.
The HCCA's are a very underrated subwoofer. They use technology from TC Sounds, 4" high temp coils, and are built like tanks.