More of the same RAM, or raise frequencies?

8gigs should be plenty for most applications. Unless you are swapping lots of textures or working with photography in one way or another.

what are you working on?

Gaming? there won't be lots of extra frames with 1600 ram. might as well not sacrifice the good ram you have.

I myself do run 16gb of 1333 and haven't even looked to see if it helps in "any" game but I do have use for it in my work. Had thoughts of running faster ram but don't think it's really worth the added expense for nothing.

look around the web for benchmarks doing what you do. bet speed or latency doesn't matter a whole lot.