1250-1600 is best (obviously) @ 1600, actually it's excellent.
Other's have pushed to 3200 with excellent results. While it's perfect at 400, I would recommend overexposing a third to two thirds of a stop when pushing, ie: shoot at 1000 pr 1250 for a 1600 push, or 2000 or 2500 for a 3200 push, it really helps with the margin of error with even great metering.
I've wondered why film manufacturers don't publish the reciprocity characteristics out to minutes. They spend all this money configuring a "new" film and leave things like this for speculation.