not true cas latency is more important than ever... if you can get your ddr3 1600 cas latency down to 6 or 7 you can outperform ram that runs at 2100+ c9 if you can get the best of both and get fast 2200+ c7 then your bandwidth will be stunning. infact it will be more efficient than ddr2 1066 running at cas 3 which is pretty damn good anyway.
the faster the ram and lower the cas will have an impact on your system performance. the ram speed will show in high end apps only like benchmarks and transcoding but cas latency will help with everything. as its not restricted the same way as ram speed.