Crucial M4 512GB Speed Issues

So, I built a new system yesterday with these specs:
i7-3930k Processor
32GB Corsair Vengeance RAM
ASUS Rampage IV Formula Motherboard
512GB Crucial M4 SSD
EVGA nVidia GTX 690 GPU

Now, it's still practically a brand new install. I'm testing the speed of my SSD with HD Tune Pro and when Origin was checking my game install.

It seems as though my maximum speed so far has been about 390MB/s. Here's a screenshot from HD Tune Pro just now: http://i.imgur.com/BFwww.png

I have it in the Intel 6Gbps port as far as I know. The only other thing connected to my mobo is my disc reader/burner in a 3Gbps port, and my WD 2TB Caviar Black in the other Intel 6Gbps port.

Does anyone know why this might be happening? Reported/advertised speeds are 550MB/s+.