The best results in your particular instance is to get a fully matched 4x8GB set. Otherwise the next best is two kits of 4x4GB DDR3-1600 CAS 9. I've had good luck with Corsair Vengeance putting together 2 kits of 4x4GB. I would suggest if you have any problems to manually set both the VCCSA & VTT Voltages to 1.10v and attempt using the XMP Profile.
If you have problems then please feel free to PM me, and I'll try to help.