The Lumix GH3, for example, has an electronic shutter, but it takes 1/10s to read out all the rows, making it less useful. Read more about the rolling shutter artifacts associated with the electronic shutter here.
An interesting Panasonic sensor data sheet has popped up. We don't know for sure, but it is widely believed that this sheet describes the sensor sitting in the Lumix GX7 and GM1. Some also speculate that it will be in the upcoming Lumix GH4.
The data sheet says that there are two options for electronic shutter readout: 12 bits in 1/15s and 10 bits in 1/22.5s. It has been demonstrated that the Lumix GX7 has a readout speed of 1/15s, and I have measured the speed of the electronic shutter in the GM1 to around 1/25s. This indicates that you lose two bits of dynamic range if you use the electronic shutter mode of the GM1. Is this a problem? I'll try to find out with a comparison.
A common way to measure the dynamic range, is to see how dark shadow details you can extract, without getting an excessive amount of noise. So the test will be to photograph a picture with a dark area, and see what the noise characteristics are in the dark areas. Here are some pictures taken using both the electronic and mechanical shutter, at various ISOs:
As you can see, the JPEG output files are very similar. You'd be hard pressed to pinpoint any difference at all.
To better see the difference, I've increased the brightness by three stops, and enlarged the centre dark area. I did the raw processing in Panasonic's own SilkyPix software. Here are the results:
As you can see, there is a more noise in the shadows when using the electronic shutter, both at low and high ISO. So using the electronic shutter has an impact on the image quality. If you increase the brightness by three stops in post processing, anyway.
And that is the point: Most of the time, you don't lift that shadows that much, and you are not likely to note much image quality difference.
When designing the Lumix GM1, it appears that Panasonic made a compromise: To avoid rolling shutter artifacts from the electronic shutter, they increased the sensor readout speed.
At the same time, this loses some dynamic range, and you will get more noise from the shadow areas. However, most people will never notice any difference, since you have to dig deep into the shadows to see any. So I think Panasonic struck a sensible balance here.
I haven't use the Lumix GX7. However, as it has the slower 1/15s electronic shutter mode, one could guess that it uses the high quality 12 bit sensor output. Hence, you should expect the same quality across the two shutter options.
Electronic shutter readout speed
Electronic shutter, number of bits
The table above summarizes the speed of the electronic shutter readout. A slow speed means a higher risk of rolling shutter artefacts.