There is definetly a sweet spot with polyfill, cram too much in and you will reduce the internal volume (I've done this at times to tighten the response of sealed subs, but you really have to cram a lot in)
Lining the walls has some effect (and is all I'd do for vented enclosures, if at all) but stuffing in loose and filling the space has the most effect.
Note that the "making the effective space larger" is really small side effect (a change on the order of a few Hz) of what fill primarily does, which is damp resonance, it's a SQ tweek, not a performance tweek.
