All I know is that the older JIUCBs weren't very good; I think they're the worst B stepping, and if not, then they're very close. Mine (0250 or 0251, I think) could only do 2ghz, and that was at 1.8-ish volts.
Pump a bit of extra vcore into that JIUCB of yours and you should reach 2ghz. Just don't expect much higher.