I've been looking at the crop factors, especially the Extra Teleconversion Mode. All the information is in the manual. My calculation suggests that in ETC mode, shooting movies in HD, where the crop factor is listed at 2.7x, you'd be getting the same screen coverage as shooting with a camera with a 1/2 inch sensor. Does that sound right?

That's right Steve,
It is actually a further 2.7x on the 2x crop of the sensor, so basically you get a 5.4x crop in that mode.
This will help a little. GH5 Teleconverter Function - YouTube