InsightIQ won't help by itself.  With the latest release of IIQ, it will report on quotas but won't do things like calculate the differences between 2 quota reports.  You can export the reports and do that manually.  What I do at our site is feed the user and directory quota information into graphite and then I can hunt down the offender or simply present utilization graphs over time. 
HI there,  I have had a look around to see if this has been covered already, apologies if I have missed it.  Is there a way using InsightIQ or any other command to see who/ip address is currently wrinting large amounts of data to the Isilon Cluster? I have noticed a 200GB drop today on our nas and want to find the culprit!  Hope you can help,  Many thanks  Jon.
InsightIQ - how to find who or what is filling up cluster space?