8-core processor bandwidth is up to 21.3 GB/s
That says that if the task is pure CPU, the 8-core is
twice as fast as the 4-core.
But if your task has to do a lot of interaction with memory (RAM or HDD Writing/Reading), the advantage drops almost to nothing.
Video editing has a lot of interaction with RAM and HDD.
Memory bandwidth is inadequate for both 8-cores and 4-core.
Memory copy speed is at best 2.9GB/sec on the Mac Pro while maximum processor bandwidth is up to 21.3 GB/s. More than double the memory copy speed.
NOTE: 2.9GB/sec is the max RAM bandwidth independently from processor's number or Ram modules installed.
HOPE THIS CAN HELP