Purifi makes "Extended Woofers", and specifically address issues that are not of as much consequence to subwoofers that don't also cover the "voice range" of 100-800Hz.
Their unusual neutral surround design eliminates the Radiating Area (Sd) Deviation (R.A.D.) that is unavoidable with a half roll surround.
Purifi does not attempt to use the same excursion on their smaller drivers as the larger ones, the PTT6.5" has 9.8mm Xmax, the PTT10" has 14.75mm.
The R.A.D. becomes relatively larger on smaller cones, which may be an issue more recognizable when comparing small drivers with the similar Xmax to larger drivers in the same "family", like Dayton's UltimaxII series.
Ease of placement of multiple smaller cabinets and the option of opposing drivers for vibration cancelling tend to favor that approach, though more than 2x12" may be required to equal 1x18" displacement.
The contribution of the surround to Sd absolutely means they are audible, and their deviations on the in/out stroke varies with excursion. Suspension non-linearities may be more noticeable than other types.
The ratio of Sd between 12" and 18" is similar to 8" and 12", but the R.A.D. increases more in the smaller Sd drivers.
Art
Someone on avsforum.com is replacing all his JBL 12's with 18's on his home theater wall. He said the a single 18 sounds better than multiple 12's.