Ideally you'd light a low light situation, not always possible of course. The Sigma 50mm f/1.4 from what I got to play with on a 5DII is stellar wide open, it'd be great on film.
I find it all scans excellent, it is honestly up to the user in that department, though hardware does let you down in the detail department unless using an expensive scanner.
Yeah I don't think you'll match the ISO 6400 performance of the 5DII or D700 etc, but given the curve published for Portra 800 in a 2 stop push, it is pretty promising.
Porta 800 should push better than 400. Portra 400 however is simply the best versatile/all rounder.
Portra 400 response:
Portra 800 response:
Vision3 500T response:
Superia 1600 response: