I just picked a random HDD, Id rather have something faster in there aswell. You could put in a slower drive with more space I guess.
And that SSD is fast enough, you will not notice something that is 500 or 575 mb/s. Yeah the write speed is maybe below 300 mb/s, but you are much more reading that writing anyway. Plus it has proven itself as being reliable and is fast enough, which is more important for me. But I guess you could also put an 840 pro in there.