Most games won't make much use of 2-4 cores. Some titles (notoriously Battlefield 4) can perform surprisingly better with HyperThreading disabled. For gaming it's really just all about raw speed and GPU frames per second. And how well the game was coded, which is beyond our control.
I wonder if your cooling block perhaps provides less efficient coverage over the physical die location of that one core?
Look at the bright side: if your part didn't have that one "bad" core, it would've probably been binned as a Xeon, lol.
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[/Korth]