I recently brewed a Belgian Dark Strong/Quad, and my OG was supposed to be 1.103, but my gravity came out to 1.087. I did forget my 1 lb sugar addition in the BK, which I'm kind of happy about because I think adding sugar after fermentation slows is a better for the yeast. Obvioulsy the lack of sugar in the boil decreased my gravity by approximately 6 gravity units, and I did have to ad 2.5lbs of Turbinado to get my gravity up to 1.103 according to my calculations. The beer fermented down to 1.020, which is about 79% AT.
Having said all of that, my main point was going to be that it is difficult to get the extract efficiency out of the Mash tun for a gravity that high. Without my sugar addition, my efficiency was ~65%, including my sugar addition, and decent attenuation my brewhouse efficiency remains at 79%.