I currently have 8gbs of ADATA Premium DDR3 Desktop memory running at 1333mhz (667mhz) in dual channel. I am wondering if I can add let's say some Corsair 16gb 1600mhz in the other dual channel ram slots on my mobo. Will it still have a performance increase like as I would adding 1600mhz to 1600mhz or will it not help?
As coastie65 said, higher speed RAM will de-clock to the lowest speed RAM module in your setup and chances are your default timings are different also. Too many variables are introduced which can cause instability in your system which is why most people will buy a pre-tested kit to eliminate those variables.
Yep, never a good idea to buy DRAM with the intention of mixing it with another set, can be very problematic....get a set of the amount you want, get it running....then..can try and add your old, use it in another build or sell it off