The 960fps requires the dedicated RAM buffer on the camera sensor, I don't think the Sirocco has this. The way it works on Sony and Samsung units is it buffers the video direct to this RAM, then it plays back the stored frames at regular frame rates which the phone can then process as video. The size of the RAM dictates the video quality recorded, higher frame rates need more frames to be stored, therefore you need to cut down the resolution of the frames to fit. While cameras like the Sony RX100 V say they record 960 1080p they actually record at a much lower res and it is scaled up, so not a true 1080p. The 720p @240 might be a bit more doable with the current hardware.
The Nokia 8 support slow-motion recording.