It all depends on the install.
If you have proper box size and tuneage, it should be louder due to more cone area.
Edit:
BLs are power hungry, so off that 1200, you're not using the BL to the full potential. I had one fully loaded on about 1400 rms, and it laughed at it.
Going from 1 18 to 2 15s will be a good upgrade.
A general rule of thumb is off the same power and in the correct enclosures, more cone area will be louder.