There was a kernel panic if a FreeBSD kernel was built with both VIMAGE and IB/OFED. This was fixed ~December 2014. Unfortunately this was too late in the release cycle (FreeNAS 9.3 was released the same time). There are no plans to release this in FreeNAS 9.3 either.. gotta wait for FreeNAS 10 I guess. Shame indeed, FreeNAS has a great UI.I tried using Mellanox cards in FreeBSD based systems such as pfSense and FreeNAS. Never got it to work, and found it easier to just go with the tested and tried Chelsio cards.
Feature #2014: Infiniband support for Mellanox cards (OFED) - FreeNAS 10 - Bug Tracking System