100MB/s is around the top speed of most HDDs when they are doing SEQUENTIAL reads/writes. Very few use cases result in purely sequential reads. SSDs are able to achieve their maximum speeds in many more use cases, including very random reads/writes. Especially if you are putting a modern SATA SSD into a PS4. Since the bottleneck becomes the SATA II interface and not the NAND flash or controller. Even DRAM-less SSDs are able to achieve very high random I/O throughput. Much, much, much higher than any HDD. Even a slightly random load will cut a HDD's throughput by 75% or more.
I can't think of a reason or situation where a DRAM-less SSD will be slower than any HDD in a PS4.