I routinely make exposures measured in the minutes. I hand hold down to 1/8s wide open in mixed lighting in city streets as well as fading light at dusk at ISO 1600 so that should give an idea of light levels. I also shoot under very flat natural lighting with extreme color temperatures--dusk in the forest type of thing. From long exposure night photography to street photography. I just made 36" x 24" prints from a file shot at ISO1600 with a minute exposure time. The print looks great--much better than I could have gotten with ISO400 medium-format film. Maybe even better than ISO 100 film. Certainly no worse then my D800, perhaps better. To me "good enough" means something you can print 3 feet or larger and hung in a gallery.
That type of performance is anything but normal . Sort of falls into the same class as asking Tiger Woods what driver I might use. I don t doubt for a minute that you are achieving such results ..just saving its not common ...so others should not take it as benchmark of performance .
I have an S2 system and have used it enough ..from landscapes in Aspen ,to shooting both action and the crowd at the US Open Tennis . Done street shooting similar (not as good) as Kurt etc .
The most common reason to invest in MF is to gain access to the superior IQ . This in general includes the ability to render fine detail,provides superior tone separation and realistic color . To gain this benefit generally it takes greater care in your technique. Using higher shutter speeds or a tripod to minimize loss of detail (which you can easily see in a MF file ) . I generally try to shoot static subjects (so its only me that is moving) at 1/2-3X focal length . Yes I can handhold the 70 /2.5 at 1/60 but I will lose some captures at anything less than 1/250 .
Yes you can shoot at ISO 1600 just be sure to add in exposure compensation of 1/2 to 1 extra EV to keep those shadows open ....isn t that shooting at ISO800 when determining your shutter speed?
Don t forget that in MF you have way less DOF ..so you get some really cool bokeh ..but when shooting 2-3 people in a group which one would you like in focus . One of the most difficult aspects of limited DOF is actually considering where to place that small wedge of sharpness.
So if you are used to a 35MM DSLR you have some adjustment to do . I try to shoot street at f5.6 never less than 1/250 and landscape at f8 or f11 which requires a tripod .
Sure I want higher ISO but generally I want ISO800 on the new S to look as good as ISO400 on the S2 . Beyond that you have to bring the "A" game and that requires not only ability but also plenty of practice .
Now I can also compare this to a D800E using the best Leica R lenses I can find . This of course is a bad example of what I started the response with . I have good eyes and used Leica R lenses from the beginning shooting plenty of sports . I would not recommend this to most photographers as its just too hard to get acceptable hit ratios. But if you are speaking of night shooting ...the D800E is stunning up thru ISO3200 and excellent at ISO6400. Smaller faster lenses and its night verse day in available light and night photography .
Generally I think of high ISO performance as ..whats the highest ISO you can routinely get excellent quality . If is 6400 is the limit ..then I look at 1600 as a pretty good indicator of what I would use . I like the S2 to about 800 but its better at 400 ....so any improvement in the S could make ISO800 a really good performer .