Sampsa got 950MHz with stock voltage, yes, BUT that was with DryIce cooling! (-80C).

Techpowerup clearly states in the article that they got 911MHz at 1.36V (default is 1.2V). 850MHz with default voltage.

Vr-Zone got 880MHz with improved default cooler + default voltage.
They did reach over 1GHz BUT that was with voltage modding (1.35V) and Cascade cooler (which means around -80C).

I dont think anybody has reached 1GHz with watercooling (with any voltage).

So your result at 875MHz with default volts is pretty good