It appears that the current REST API may not support performance and monitoring information. I did find in the CLI Administration Guide details on extracting performance metrics; while not ideal compared to REST, you may be able to still automate collection of performance metrics. I also recommend placing a feature request for performance related commands in the REST API.
Like Ryan said in the REST API there is no performance data.
This would be nice to have, and the Array temperature would be nice as well.
Currently I use a combination of SNMP and REST API for monitoring, but I found that SNMP is far form perfect as we have lots of volume creation/deletes and the index keep changing.