Yup - OIS for video and photos only on the 6S Plus. Was thinking along the lines of shot stabilisation on FCP X can use the larger image. Do we know if the 6S Plus actually shoots above 4K, then can use this leeway to stabilise the video for example?
8MP is 3,264 x 2,448, so it's already well above 1920 x 1080. The quality of the camera itself such as the sensor/lens/software will make a difference, but not the fact that it's 12MP vs 8MP.