85mm 5.6 lens and 42.5mm 2.8 with same diameter of aperture.... that's right because this is how the aperature value is defined.
A 85mm lens at 2.8 may capture 4 times as much light as 42.5mm at 2.8 but at the same time this light is distributed over 4 times as big an area so to each pixel of each respective sensor still get the same amount of light on it if exposed for same shutter speed, same iso and same "aperture".
It is not the same as saying the smaller sensor has to bring the brightness level up with less total light captured ?
I suppose a hazy day is a good choice. Sunny day may post huge even un-workable stitching problems ?
New if they can steer clear of this mess and instead to do a full frame (Sony R1X) with exotic Hassy lens, remove the AA filter ....