The sensor delivers one stream of raw images which includes 8mp @ 15fps or 1080p @ 30fps.
Those raw images have to be processed before they are viewable. That processing is done by a hardware block called the isp (image sensor pipeline). It can process around 100Mpix/s, therefore 8Mpix15 exceeds its limits before you even think about your 1080p stream.
You can stream 8Mpix at 10-12fps, but only if you use mjpeg as the codec. The H264 encoder is limited to 1080p.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
I'm not interested in doing contracts for bespoke functionality - please don't ask.