I have seen this wear on almost every well used BP style milling machine I've ever seen.
It comes from the table overhangs and running the lead screw to each extreme, putting more pressure on the outside ends of the saddle dovetails. This produces a crowning of the saddle dovetails, and then the cast iron table 'relaxes' due to gravity to conform to the crowned saddle.
In all my years machining I have never seen any BP style mill suffer from over-tightening vise studs in a way to bow the table, It would have to deform the underside of the T slots enough to move from elastic deformation to plastic deformation - at about 60,000 psi Since the majority of the force is in a conical strain zone, that means putting around 40,000 lbs or more of tension on the studs. I'm guessing over 300 ft-lbs of torque on the nuts, or thereabouts. The maximum recommended torque on a 1/2-13 is somewhere around 85 foot lbs.
