From the sims, my EPI 8" drivers definitely will work better in IB in the door, than in any sealed enclosure I can fit in there. As was said before, it's a matter of the parameters (Thiele-Small) of the driver. If it's what would be considered a "high-Qts" driver (Qts of .475 or higher, like my EPIs), then it's likely to work better in an IB setup. If it's "low-Qts" (lower than .35), then it definitely will like a sealed enclosure.
To a lesser extent, it also depends on the Vas (equivalent volume) of the driver. Low-Vas drivers, even with moderately high Qts (say, Vas of less than .5 cubic foot and Qts of below .45) will usually work well in sealed enclosures. OTOH, high-Vas drivers (over 1.5 cubic foot, for a midbass) will work better IB (in a door) even if the Qts is lower (at least about .35, though)...
If you're wondering wht to do with a particular driver, just get one of the box calculator programs like Unibox or the like. Put in the parameters, and "estimate" the total volume inside the door as a volume, vs. what sealed enclosure volume you can create inside the space. Compare the response... which ever one is better, that's the way to go, usually...
Regards,
Gordon.