Well, first off see section 1031 at the site I linked to above (search for Purgatory). The concept is that Purgatory is a final purification of the elect, as distinct from a the punishment of the damned.
Secondly I do NOT believe it follows that just because a Christian is in "imperfect communion" (RCC legalistic term, not my personal view) that they would be more or less likely to end up going to Purgatory. To clarify see section 837 on the page I linked to which describes full members of the RCC but warns that they will not be saved if they do not persevere in the law of charity. Now, of course, this could get into the whole works vs grace debate, but the original question was not how one is saved but whether RCC membership is essential to it, and I think to the Catholic the answer is no.