I haven't done any testing on the new Sandy Bridge CPUs but when you click on the up gadget beside the Set Multiplier box, it is supposed to show the word Turbo when it maxes out which indicates you are telling the CPU to use full turbo boost. Something could be a little different in Sandy Bridge so I might have to wait for burebista to do some testing for me when he gets his new rig going. What multiplier range does ThrottleStop let you select?

Did you try increasing both the TDP and TDC values? This will work even when ThrottleStop is only in monitoring mode. You can also adjust the Force Turbo TDP/TDC number in the Options window to a low number like 8 to see if that helps. It sounds like you are hitting the maximum TDP / TDC limits for this chip and are being throttled back.