In fact, on 2.5 inch HDDs it makes no change at all. The drives are to slow to get the benefit of 300MB/s more than with 2.0. It only makes a change on SSDs, but I don't think that the slim comes with an (big) SSD. The maximum speed of HDDs is limited for 2.5 inch drives at about 200MB/s if buffered. So SATA 3.0 will not make the drive faster. For the PS4 Pro the effect is much bigger. The use of a SSD makes more sense.