You don't really want photos at ISO 409600 per se, though, you just want photos at the lowest ISO that can get you the necessary depth-of-field for your shot (here extended with tilt-shift lens) at whatever shutter speed avoids being too shaky. And then you want as high of a dynamic range of possible, so you can distinguish the brightest brights from the darkest darks in a single shot.
If you can do that already with the sensitivity you've brought, higher ISO doesn't buy you anything...
If you can do that already with the sensitivity you've brought, higher ISO doesn't buy you anything...