Try adding some yeast nutrient. For really big beers, it can help eek out those last few points.
Try mashing the grain at 148 for 30 minutes and 154 for 30 minutes.
Sugar can inhibit maltose fermentation at high levels, so for really big beers with sugar, the incremental feeding lets the yeast eat the maltose and then later eat the sugar. Not sure what your setup is, but it'll also help prevent the krausen from coming out the blowoff. I've noticed on my big belgians that when I add the sugar after fermentation has slowed, it makes a huge krausen and will still come out of the blowoff hose even in a 6.5 gallon carboy.
I've never actually used extract, so I will defer to others for that.
Merlot Barrel Russian Imperial Stout
Vanilla Belgian Stout
Tripel w/wine tannins