If HyperThreading would have any effect on overclockability it would be slightly detrimental. Don't forget what it does: it adds extra meta-computations and loads the CPU to the max whenever instructions allow so. This means HT *increases* the stress on the core. All in all this would show in a lower apparent (surface) stability in normal usage.
As far as the 3.06's stepping is concerned; it's still a C1, but it has undergone a slight change from fixed VID to multiple VID. From glancing at Intel's PCN docs it would seem that this simply allows for a more variable core voltage specification for these chips. Maybe it will result in more voltage-induced headroom in some, but it may also starve others AFAIK.