The issue with this setup that you want to do is that it's just overcomplicating and also you'll have to take account of the actual sata 2 speed limitation of the mac pro internal connections. 300MB/s each sata slot. Also, those docks would cost you at least 50-100USD each depending on where you buy them, so you'll have to add that to your build cost. Having fusion drive as you already have mentioned is asking for trouble without raid1. I might be speaking for myself only but i think people appreciate things that just work without having "failed drive volume" in the back of their minds all the time, since not everyone wants to deal with raid1.