Wasn't thinking much about meaning, more about form :-) I can't see any nuances in meaning that differentiate these two prefixes, hence I'd have to ascribe similar meanings to imperfectibilby and its un- cousin. I can think of differing meanings I might like to allocate, but that's not the same issue. For example, I can conceive of:
- unable to be perfected because of an inherent deficiency in the agent attempting to create perfection
- unable to be perfected because of an inherent deficiency in the object of the attempt to create perfection.
A curious pair: unperfectability and imperfectibility. The first is non-standard, I think, and comes from the English verb "to perfect" (stress on 2nd syllable): perfect + -able (which can be added to almost any transitive verb to produce an adjective) + un- (which can negate pretty much any adjective in English).
In fact, you probably won't find the perfectly plausible perfectable in most standard English dictionaries, and that's because the Latinate perfectible is so well-established, that the -able ending registers as incorrect. Because it derives from Latin, "perfectible" yields the equally Latinate negative imperfectible and hence, imperfectibility.
The variant you suggest, Pro, is not implausible, but because it combines the handy English un- prefix with the Franco-Latinate -ibility suffixes, it seems strange.
Kushner's word unperfectability means essentially the same as imperfectibility, but it has a somewhat ad hoc feeling, representing an instantaneous thought process (is this Belize speaking?) that goes something like this:
1. No one can make the world perfect.
2. The world is not able to be perfected.
3. The world is not "perfect-able."
4. The world is un-perfect-able.
5. The world is characterized by unperfectability.
By having his character say "unperfectability" instead of the standard "imperfectibility", Kushner conveys that the character is thinking in concrete, rather than abstract Scholiastic, terms. Which is sort of the whole point of the Belize v. Louis contrast in the play.
The distinction Bilby suggests has merit, too, I think. The word "imperfectible" probably developed in medieval religious thinking with regard to the question of the perfectibility or imperfectibility of man, given the doctrine of Original Sin. So it does point to an inherent deficiency. "Unperfectability", on the other hand, is, as I suggest, an ad hoc term related to concrete thinking ("Can we perfect this situation? No? Well, okay. How about this other situation then? Maybe we can perfect that. Let's give it a try.") and is not really concerned with inherent deficiencies, only with possibility or impossibility at the moment.
Oh, Joe! Well, he's another concrete thinker, despite being a lawyer – unlike his Antarctica-roaming wife.
By the way, I don't think I would ever interpret "imperfectible" as meaning "capable of being made imperfect" because there is no verb "to imperfect". I might say, rather, something like "blemishable" (a word that I doubt exists in dictionaries but is perfectly understandable).