The 35mm f1.8 is indeed very sharp, and it is very sharp from f1.8. Quite surprising for a lens of this price, as you don't really need to stop down to get the great sharpness.
With the wide aperture, you will minimize the need for the Nex 7 to shoot at high iso thus avoiding the only issue of the Nex 7 in terms of noise at high iso.
Observation: all Auto AF, Auto ISO, daytime shots stays at ISO100, which indoor night shots stays at ISO320.
Sigma 30mm is also a good lens, but at f2.8, you will need to pump up the iso for lowlight shots to prevent blurred shots, and thus creating high noise. The built quality is all plastic, and has the "famous" sound in the lens when not in use. This 35mm F1.8 is very well made, with solid feel.
AF is very very fast with this lens with totally silent focusing. I get more keeper shots with this than my previous X-E1, with almost all shots are sharp and correctly focused with the AF, as compared to my X-E1 which gives me more OOF shots due to its AF. Not complaining about the X-E1, as it takes really sharp photos when the AF is accurate, only when the AF is accurate, however. Or you have to rely on manual focusing.