CAS 3 is a little bit slower than CAS 2.5. On synthetic benchmarks, CAS 2 is a lot faster than CAS 3. CAS 2.5 is somewhere in between (although closer to 2).
I'm not sure about real world performance though. There is probably not a great difference in performance.
label says 3200 and it only works at 2700 speeds? Where did you get it? I would return it and get a refund. Another think about a lower CAS latency (3,2.5,2,1) is that it is better for overclocking. Typically better ram all around.