The speeds on their specs are benchmarks under Windows. I have a Vertex 4 and it only gets around 330mbps write, still the fastest of all my SSDs. I had a Corsair Force 3 GT in a 2011 MBP, it got good read speeds, but the write was similar to what you have. I put the same drive in a windows machine and got the benchmarked speeds. Do you have TRIM enabled? I know that has a big effect on the write speeds too.