It was known for a long time that the "incompatibility" problems with Yorkfield in most cases are just signal level problems. So, depending on the signal level tolerances of your particular board AND CPU, the results can vary from "not even stable at stock speed", to "stable when slightly overclocked", as in your case.
Unfortunately, these signal levels can't be adjusted through BIOS in the majority of older boards. That's the reason why the manufacturers said that the boards were not compatible. Just to be on the safe side.
As I already said, if you were to manually tweak those control voltages, then you might gain stability at much higher clocks. The question's just if it's worth the effort in both time and money. It would involve a serious amount of SMD soldering, which sometimes can be a real pain in the... Well, you know what.