One thing to consider is that 400 is only two stops brigher than 100 (3200 is 5 stops, so thats a bit more usefull). So you're only opening the shutter one stop, and the apperture one stop. In most cases this isn't a problem. I wouldn't describe my stuff as street, but I've not had to many problems getting enough on 100 outdoors (dark alleys excpted). No, interesting things don't always happen in bright places as noon, but you've got at least six stops below that before you start running into serious problems.

Of course loosing two stops on the apperture makes your focusing a lot more critical, and often you don't want to be messing with focus, and mess a great expression. In those cases, I'd just set the focus to 2 or 3 meters, stop down as far as I could, then figure out the depth of field I had, and be ready for people to walk into range...

Hey - get out there and try it. If it doesn't work, try something different...

Ian