I updated to 5.6, and I'm all ITB, with very heavy use of Kontakt libraries (Albion, Da Capo, Grosso, Tina Guo Cello and Action Strikes, in particular), and I haven't yet noticed any issues with drop-outs or crackling.
I've been lucky, I guess. Normally, I'm the dude who gets hit with the upgrade bugs, crashes and esoteric error messages. But aside from my dislike for the new keyboard colour system, I've really had no issues with 5.6...