I've pushed Tri-X 400 (6x6) to 12800, once for a model shoot (in a setting where grit and tons of grain were very appropriate) and once for a still life. Loved it. I lucked out, particularly on the model shoot, because development was done by total best guess. Contrast and sharpness were good.
I've never tried pushing 3200 to 12800, but I suspect it would do pretty well -- if shot in very flat light.