If it were me (and it could be, as I haven't really put my D800 through its paces yet), if my outermost AF point were 80% accurate, I'd declare victory and use the camera without a second thought.
Phase-detection AF systems are inherently a set of compromises. With 51 focal points in the camera and countless lens designs with differences in field curvature, chromatic aberration, off-center resolution, etc., it seems to me that Nikon prioritized this right in trying to get the center dead-on balls accurate (it's an industry term) and letting the less accurate outer points do the best they can.
(And yes, I have heard that Canon has better AF on their bodies. Maybe that accounts for the cost of the 5D Mark III: the extra $500 gets you better AF fine-tuning. )