Just out of curiosity and I can't seem to find this information on the web, does anyone know what is the actual number of pixels used for, say FHD 1920x1080 video. If the video size is 16:9 and that's cut out of a 4:3 sensor, it's hard to think that the full 16.5MP sensor is used.

The camera down samples the full sensor (except for what doesn't fit in 16:9 ratio). It is probable that the camera is using the full width of the chip (or close to it). You could take a photo in 16:9 and then check it. However when you activate the ETC you only use the exact pixel count for HD as the camera is only recording the image from the center of the imaging chip.