First of all try to identify what can be the possible bottleneck of the decreased performance.
It would be good to check your H/W RAID settings.
The following Knowledge base article should be helpful for you:

Information on Configuring for Optimal RAID Performance

System performance, and RAID BIOS settings.

After you are sure that ther RAID configuration is optimal, it would be good to make some read and write performance test on DSS V6 to confirm that the RAIDs performance meets your requirements.

Our webcast about performance testing should be helpful.
Performance Testing with DSS V6 for your NIC's and Volumes:

Finally you may take a look at another KB article:
How-to dramatically improve IO performance?

It may be also needed to update the FC HBA firmware or ewentualny try to apply newer drivers for your FC HBA to DSS V6.

You need to make sure that each part of your hardware are working at optimal speed and is correctly configured before you can blame the software for below average performance.

Best Regards,