They're dropping 2 guaranteed rares to 1 with 5NV stacks on the bosses.
To counteract this, they're guaranteeing each champ pack to drop 1 rare when you have 5 stacks.
I think act 1 will be the place to farm once it can drop ilvl63 gear. Huge concentration of easy champ packs that guarantee 1 rare each. I can clear warden/butcher in under 15 minutes with MF gear.
Once 1.03 rolls in, I'll probably just clear everything in cemetery, festering woods all the way through...