you get what you pay for. Server level expanders use all four channels at once, expanding to 12 or 16 channels. With this, you'd be narrowing down 4 drives worth of bandwidth into a single channel. Performance would suck eggs. Oh, sure, you'd get more drives to work with, but the performance would be far less than it could be.
Let's see. 16 drives is 400% more than something. So something = 16 / 5 = 3.2.
Previous generation hardware supported 3.2 drives? Or has the nonsense introduced into our language by innumerate and practically illiterate ad writers spread to people who can actually count? It's 400% as much, or 300% more, or even four times as much if you are writing for people who understand numbers. I'm going to assume that this was taken from the mfg's copy, not written at Tom's.