Nobody knows for sure yet. In theory its doubtful it'll scale linearly with all tasks because your software will split the workload between the CPU, GPU and the prores encode/decode engines and it you may not need all the resources at hand.
I notice this watching my Afterburner in the Activity Monitor - with 4k prores it kicks in a bit but never tops out, so there are spare resources at hand while the GPU is maxed out.
I would go out on a limb and say that not many people really needs 4 encode/decode engines, but they're already on the die so double the die = double the media engines.