Update :
Doing what you said, I can easily achieve 2,6ghz. Still, I can't achieve 2,7ghz, and 2,65ghz is extremely instable.
Using Prime 95, Core 2 fails the test at approximatively 3 minutes, where Core 1 seems to go on forever.
Temperature goes up to 55-56C when both are full load, and drop to around 51C when core 2 fails.
Vcore is about 1,47-1,49V.
I hope I didn't fall on a crappy X2 :(