The HC-W850 and 750 are the same camera, the only difference is that the 850 has an extra camera mounted on the side for PIP footage, that is where the extra cost comes from.
The sensors those cameras use are new, have a high resolution and use a new processor. I think it is likely that they are going to use that chip set in much the same way Sony used the RX10 chip set in the AX100 and CX900. So expect to see it used for 4K in a 950 and video capable point and shoots as well.
The 920 does not have good image quality. It is not that great in low light. Having three sensors does not mean that it has higher light sensitivity. Although it has high resolution the compression factor is too high to handle it cleanly, so you see a lot more artifacts in raw footage, such as smudging and smearing. The new Panasonic cameras shoot at up to 50 mbps. They probably also use advanced sampling/downsizing much like the Sony cameras do.
It would be interesting to see some raw footage from the new chip sets. There are only a few videos on Youtube, but they show the effects of heavy compression, so they are not fair assessments.