>The real point of the SPLR 8" vs the SPLR 15" is that the same vendor wants $1600 for the latter. Three smaller 8" subs offer just under the total radiating are of the single 15" sub (if my math is right) and my guess is that distortion levels are slightly (i.e. proportionately)higher.
You're interested in displacement not surface area.
Displacement is the product of surface area (maybe 200 cm^2 and change for cone + 1/3 of the surround on the 8" driver versus 800 cm^2 for a 15" driver; meaning one 15" driver has a 12dB output advantage or you need 4 to equal it) and excursion where the geometry of a smaller driver imposes limits. You might get 12mm of travel with the voice coil completely in the magnetic gap on an 8" driver (Tang Band's 8" sub-woofer) and 23mm on a modern 15" driver (like the AE15 bringing he total to nearly 18dB more output from one 15" driver with 8 small sub-woofers needed to equal it.
You're interested in displacement not surface area.
Displacement is the product of surface area (maybe 200 cm^2 and change for cone + 1/3 of the surround on the 8" driver versus 800 cm^2 for a 15" driver; meaning one 15" driver has a 12dB output advantage or you need 4 to equal it) and excursion where the geometry of a smaller driver imposes limits. You might get 12mm of travel with the voice coil completely in the magnetic gap on an 8" driver (Tang Band's 8" sub-woofer) and 23mm on a modern 15" driver (like the AE15 bringing he total to nearly 18dB more output from one 15" driver with 8 small sub-woofers needed to equal it.