It's normal for the board to see your memory as 1333MHz since that is what the memory controller supports up to natively.
When you changed it to 1600, did you change the memory timings and voltage? Since you're running 4 modules, you may have to increase VCCIO a little.
|Intel i7 2600k 3.4ghz Sandy Bridge||Asus p8p67 Deluxe||AMD 6970||Corsair Vengeance DDR3 1600 16gb|
|WD 1TB 7200RPM, WD 500GB 7200RPM||Windows 7 Pro||Dell Ultrasharp IPS 24"||Corsair 750tx|
|Antec 1200 v3|