The change to P20 came with 9.3.1. Wasn't that released late June?
I don't recall exactly, but the point is that for awhile after that change was made, things were quite rocky because there were some people who had upgraded to 20.00.02.00 or 20.00.00.00 and this was causing some of them severe problems. I believe other people continue to have problems such as the apparent dropping of support for SATA 1.5Gbps (SATA I) in all versions of 20.*.
As is my general tendency, I like to let others discover the faults in software, firmware, and hardware rather than just trusting blindly that things will be fine. As a result, since our P16 stuff was running fine and there was no pressing reason to upgrade, and since there had been some Significant Issues with the P20 stuff, I placed a six month delay on upgrade plans back in October. That recently expired and yet because I've got too much to do (am lazy) I still haven't gotten around to it.
To me, there's a lot of value in running known-good versions of things because I already have enough engineering fires to deal with. That's one of the reasons we avoided SAS 12Gbps for FreeNAS on our last round of FreeNAS boxes; the ~millions of hours on LSI3008 pales next to the ~billions of hours on LSI2008/2308 on P16/P20. I *just* *this* *week* finally killed off our last two ESXi 4.1 hypervisors in the office cluster in favor of a particular version of 5.5 in order to avoid another dependency hell.