in synthetic benchmarks there's a difference but in most "real world" type benchmarks / apps the difference is pretty negligible - if you already have the 1600 it's not worth upgrading at this point, lower CAS 1600 is better than high cas 2133 or 1866
I went for 1866 purely because I found a good deal on 1866 cas 9 and most of the not outrageously expensive 1600 is also cas 9 so the bump to 1866 was effectively "free"
you could just try getting your existing RAM to run at tighter timings possibly with a small voltage bump (after all, what's the point of those massive heat sinks if you can't overclock it in some way)