In addition to not really being designed to belt out much below 30hz, pro audio subs are also designed in such a manor that they really rely on the enclosure to provide their low frequency output and extension (ported, horn loading, etc). And in a situation such as this, excursion isn't necessarily the dominating factor.
Less excursion = less distortion, for any subwoofer ... in relation to itself. Subwoofer "X" will have less distortion at 5mm excursion than it will at 15mm. This is true of any subwoofer. However, subwoofer X won't inherently have less distortion at 5mm than subwoofer Y at 5mm just because it has less linear excursion capabilites (Xmax). Quite the opposite will generally hold true. If subwoofer X has an Xmax of 8mm, and subwoofer Y has an Xmax of 20mm....generally at 5mm (just as an example) of excursion subwoofer Y will have less distortion since it is operating further within it's linear excursion capabilites, wheres subwoofer X is operating very close to it's linear excursion capabilites. The closer you are to Xmax, the higher the distortion. This is one of the benefits of a high excursion driver, even if you aren't going to actually use all of it's excursion capabilities.
Obviously the above isn't all encompassing, and things like motor design, etc will affect ultimate distortion and "sound quality" performance. But, as a generality....