Edit: Maybe this is a silly question, but it is possible to build the FreeNAS kernel and the ZFS tools (zpool, zdb etc) with debugging symbols? Would this be any help?
If you have to ask if it would help, the answer is "it won't be useful for you and your situation". That's for programmers that want to debug the code. I can pretty much guarantee you that the freenas devs don't really care about debugging the zfs code since that's not their baby.