I don't think there is a sweet spot for
cone excursion per-se, but will depend on a few factors -- what type of enclosure are you using -- i.e. Sealed, Ported, Reflex, Coupled,
etc. -- range of the full
system being another, and desired
level of playback.
You're going to inherently get a large amount of phasing smear when using subs, which is partly why an acoustically coupled box has advantages (better efficiency, cones exert less with the same dbSPL as a single
cone, thus phasing smear is reduced). If your aim is a loud, clean, and coherent
system then you need efficient boxes that can exert the cones as close to center as possible, and splitting up the work among many drivers is the best way to do this. The more you have, the more you open yourself up to
phase issues, but the reason you often see 2x drivers in large enclosures is mainly efficiency and this efficiency translates into better
phase coherence at relevant SPL levels to a single drive enclosure.
Long story short, you can't look at an individual
driver in a sub setup, but at the desire of the
system and the
subwoofer sub-system as part of the whole for quality deployment.
