Exactly how a sub should look...lol
So lets say 31mm x 1.0 (just a reference number) = 31 mm^3 of displacement x 2 = 62
Then 58mm x 0.9 (90% of SD) = 52.2 mm^3 of displacement x 2 = 104.4
Around 1.8 dB additional output when using the most of both. Even dropping to a more reasonable usage of, say, 40mm of travel...
40mm x 0.9 = 36 x 2 = 72 = 0.4 dB of additional output.
Doesn't take much to get a gain even with a bit of drop in SD
With all that being said -- theory only takes us so far and we've seen larger gains in output compared to theory in initial testing -- We have already made a "Hi-X" version of the Z v.3 12" model and have seen 1 dB gains on the meter vs stock models... a wider than average surround is used on them as well with an SD drop from stock cones -- and they don't gain nearly the throw.
Theory doesn't hold up as you also have the non-linearity of the surround coming into play as well as the spider... and we mate these big surrounds with much large, more linear spiders than the small surrounds -- so it's not all apples to apples in the whole driver package