The A7S / FE 24-70 is a potent combo. Images look really good across the frame at 24mm and are exceptional at every other focal length, including 70mm. I tried 4 samples of this lens with my A7R and not one was sharp across the frame at 70mm (at any aperture).
Here's some test shots from the extremes of the zoom range (all shot in raw and processed in LR5). I haven't bothered posting samples around 35 and 50mm since the lens performs superbly on the A7 and A7R at these focal lengths just as it does on the A7S.
A7S + FE 24-70, 24mm @ F8:
Link to full size: https://db.tt/qZS2bHBf
A7S + FE 24-70, 70mm @ F4:
Link to full size: https://db.tt/V3648MqC
A7S + FE 24-70, 70mm @ F8:
Link to full size: https://db.tt/R57R9KV7
I no longer have my A7R but I've been able to dig out an image taken in virtually identical conditions as the above image. These are crops from the top right corner with the A7R image down-sized (prior to cropping) to the resolution of the A7S.