DxOmark tested the Panasonic S5 sensor

DxOmark conclusion is:

Given the smaller and lighter body, not to mention the price difference, the Lumix DC-S5 looks like a highly tempting alternative to the Lumix DC-S1 for those on a tighter budget. However, while the Panasonic Lumix DC-S5 is a very capable performer, with practically the same output as the excellent Lumix DC-S1, it has slightly higher noise levels and fractionally worse low-light performance. That’s all relative, of course; the Panasonic Lumix DC-S5 is still a superb performer and remains an excellent choice for a wide range of genres.