Just because it is the same load to all drives doesn't mean it isn't too much.
Figure each disk can do 50 IOPs with good response time. Each host write = 6 disk IOs. So you can figure out how many SATA disks you need pretty easily for your specific workload.
Do you use virtual provisioning in your environment? And how are the metas configured? Striped or concatenated? What is the predominant RAID type?
Based on your answers for the above, I can may be suggest a few things, but my suggestions would really be more of the best practices, I'd recommend you engage a perf specialist from EMC.
Yes, all the drive types in the screenshot are raid 6. Even with FAST working optimally, the server teams are complaining of high response times.
There is nothing wrong with using RAID6 that adding more drives and/or DAs won't fix.
However your performance issue could be something else. Again, this should be dealt with via the SR process along with the local performance folks.