Today after reinstallation of system on my miner the AMD GPU Clock Tool which previously was the only tool that could underclock and overclock over the BIOS limits my reference HIS 5850 refused to work.
The drivers installed were newest beta Catalyst 11.7. Then I've tried to use ClockTweak to do some overclocking in the BIOS limits range, but accidentally I've passed options for my second card - 900/300 and it worked! Not only the BIOS limits were bypassed, but previously this card could only go stable to 850. Very nice job AMD with that drivers