The PS4 has two buses called onion and onion+ (stupid names lol) the big fat Garlic Bus that serves the GPU and the normal 20gb/s bus that serves the CPU alone.Possibly, but how(is?) accessing that pool of unified memory effected by the differences in HSA bus speed(I don't know)? 30GB/s vs 20GB/s sounds significant, but I must confess I don't have the vaguest idea if that has any practical effect?
From what i can work out is the onion buses share 10GB/s each way, one is a direct link between the GPU, CPU cache and the GDDR5 memory pool, and the onion+ bus looks to bypass the GPU cache. This could mean in many circumstances the CPU bandwidth would be approaching the 30GB/s, looking around though it seems the max thruput for the CPU is around 17GB/s though. The highest bandwidth is resevered for the GPU as obviously that requires as much BW as possible, but it also has direct access to the CPU vi the onion bus, this may help when the CUs are being used to help the CPU.