Does Complete Disregard check a creature's power or base power?
No, Complete Disregard won't work (and incidentally you can't target the Impostor, since it's not a legal target). Complete Disregard, like almost* all other cards simply cares about the actual power; the base power doesn't matter. Both buffs (+2/+0 from Cleaving Sliver and the +1/+1 counter) alone would be enough to raise the Impostor's power above 3, so that it ... wait for it ... completely disregards Complete Disregard.
*: the few cards for which this is not the case explicitly mention 'base power' and/or 'base toughness' (example), and as @murgatroid99 notes, they all change the base power/toughness, they do not check it.