Turns out hotfix 11 *did* fix the issue for me. Just ran all over the Lower City on foot, did some fast traveling and entered some named buildings (all things that would've caused the game to hang before) ... and everything seems to run as smoothly as before.
Performance still isn't what it is in Acts I and II, but it's totally playable again for me now with no lags/rubber-banding or anything like that.
S.
