Note: 25.6 GB/s Max Memory Bandwidth   

Thus my calculations show we are using about 25% of the MAX theoretical memory bandwidth.   Close to 50% of the practical memory bandwidth.  An ASIC would be limited to perhaps 4x gains if all they did was replace the CPU and perhaps 10x gains if they used very special purpose state of the art memory.   Of course, I doubt they could do these things in a machine costing less than $4000 so there is little cost/benefit to creating an ASIC for this