The octa-core processor being rumored, is not 8 cores at once. It is 4 low power cores and 4 high power cores. The low power cores can handle most general stuff super fast, while using significantly less power than the dual and quad core chips found in different versions of the s3. As such, the octo-core chip can offer significant power savings. However it also has powerful cortex a15 cores which destroy the quad core s3 in speed, and double the number of cores of the dual core s3, while being slightly faster clock for clock.
So basically octo-core is 4 cores at one time. 4 power saving, or 4 super powerful. Only 4 will be active at once from what I've heard, and overall there should be significant power savings.