We don't know the cause of the apparent degradation, so we can't establish a "safe" profile. However, there are things you can do to make sure the CPU is operating within the limits recommended by Intel. Start by updating your BIOS and then check all the settings below to make sure motherboard is configured properly.
You can use the values in the "Performance" column for a 250W configuration, but you need to make sure the current limits are also enforced (ICCMAX). Also make sure that CEP, eTVB, TVB and TVB Voltage Optimizations are enabled. The last few settings are essential for making sure the CPU uses less voltage under light loads.
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