In our lab, the 520 series read speed was measured at 409 MBps.
The following was used to gather data:
Created a 64 GB partition for Writing/Reading & installed Fedora 17 on the remaining space of the SSD
Measured at various block sizes from 64kB to 1024kB
Wrote/Read various sizes from 2GB to about 34GB
- 3.0 Gbps
- 6.0 Gbps
Let me know if you are still having issues.