AFAIK Smart Guardian reads the socket temp, and not the "on-die" temp. That would account for the huge difference.

As for the Xigmatek, have you checked the contact on it? I had a HDT-S1283 (which I beleive is the big brother of your heatsink) and was not getting as good of temps as I expected after seeing other reviews say it was a top performer. When I unmounted it, I saw why. Since the heatsink has direct contact with the heatpipes, there are grooves in the aluminum base for the pipes to fit. It seems with the one I got that the grooves were too deep. The aluminum base was making perfect contact with the IHS, but the heatpipes were not. In fact, such poor contact made the thermal compond not spread out as much as it should.

Unfortunately, the only real way to find out if this is the case for you is to remove the heatsink and check the contact.