You can run it at 450MHz but that doesnt resolve to 9GH/s for me. Till now i didnt find a setting that can do this. And it looks like the temperature the chips have are changing the needed mV to get the best Hashrate, so in theory each Temperature should run its own mV to get the maximum possible hashrate.
have you added the --queue 4 setting to the command line?
That is important to get close to theoretical performance.
My tests showed hashrates very close to 9ghash @ 450mhz for a single board.
Didn't test high OC with a cluster yet though.
I really have to do a MAX-OC test with a single line of chips to find out the real limits.
(maybe even more then 500Mhz)
OC headroom is also chip-dependent, some chips are just better then others.
(One reason why i want to push for individual-per-board OC settings in future firmware revs)