No answer from Netapp else than "the MIB are public, so deal with them or use ONTapp API".
We stopped using observium for the system part (servers, os, storage, FC switches etc...) because support for theses devices is clearly better on other tools.
We only use observium for the ethernet network part now, except for Juniper EX (high CPU caused by SNMPd) and Nexus1000v (same problem, with SNMPd process crashing).
For Netapp we switched on OnCommand Unified Manager and Oncommand Performance Manager, which are free, and use OnTapAPI for the polling. Clearly SNMP is not the best solution for Netapp, as we had performance issues (high CPU caused by SNMPd) on low level devices, and now that we use OntappAPI, we have an average 5% CPU on theses devices.
APIs seems to be the futur for monitoring, and SNMP is clearly not push by a large number of vendors (Cisco, Juniper, Arista for instance).
Added/fixed in r7542.