I have shot 800z at 1600 and dev'ed for 3200 and it looks good, at least in 645 format.
The slide film provia 400x also works well to those kinds of speeds.
Ultimately, though, one wants to use a combination of fast lenses, modertately fast film, and clever attention to the direction of ambient light... i.e. know which shots not to take and how to gently coax your subject into the right spot. You don't want to try to do it all with only one of these factors. It takes all of them to get good available light results.