While I won't pretend to understand all the technical details, it is my understanding that fragmentation on ZFS doesn't mean quite the same thing that it does on other filesystems, and---I don't recall exactly where I saw this---fragmentation on a pool shouldn't start having a problematic impact until it reaches around 80%.
Just for the sake of illustration: I created two brand-new pools on two new disks one week ago today. The two pools report 31% and 52% fragmentation, respectively. The pool with 52% fragmentation is about 2.7TB in size, and is 68% full. In the week since it was created, the pool has seen perhaps 60 GB of writes. Meanwhile, the pool with 31% fragmentation is only 20GB in size and is 60% full, has seen probably 40GB-50GB of writes---at least twice its total capacity.
So the smaller, more active pool is less fragmented, and both pools are only one week old. It seems the FRAG value cannot represent the same sort of file fragmentation that degrades I/O performance on other filesystems.
