The optical quality should be excellent. And you could mount an a-mount lens like this on a Sony a7s if you used an adapter such as the Sony LA EA4. However, this lens is extremely large and heavy, so it might seem physically awkward to use a small camera like the a7s mounted onto the end of such a large, heavy lens. Also note that this lens doesn't include any built-in stabilization -- that's not a problem when you use it with Sony's a-mount cameras that have in-body stabilization, but if you want stabilization in the a7s you'd be better off with a lens with OSS built in.
If you don't mind getting f/4 instead of f/2.8, then to pair with the a7s you should consider getting a Sony 24-70mm f/4 Vario-Tessar T* FE ZA OSS Lens-- it was made to natively mount to the a7s without an adapter, is smaller and lighter and cheaper, and has built-in OSS.
Thanks again. I currently use a Nikon D90 and a Leica Dlux 6. The weight of the D90 and the 12-24 lens helps to keep that kit on the shelf most of the time sadly. I love the Leica and also use an older panasonic lx5. I read about the a7s and am wondering about switching. The report of slow shutter and very few lenses has me still looking. My perfect camera would be very low weight and pro lens.