Nice... but does a 181 lb. Safety have enough sand in his pants?
Diggs is 200, Earl was 202, Adams is 215, Lott was 203, and Polamalu was 207.
You nailed it. He isn't nearly big enough to play run support in the box. He would get subjected to a needless physical beating. I don't know how his speed compares to Earl Thomas, but I could see Witherspoon raising similar hell in the "Robber" mode as a free safety. I mean if you wanted to waste his talents at safety, he would probably be much better suited for that role. Seattle doesn't play that style of defense as much anymore though.
As a slot corner, he would be in much better position to anticipate. Seattle has a notoriously bad historical track record of covering backs out of the backfield. It goes all the way back to the LOB days. Their loss to the Patriots in the Super Bowl can be directly attributed to wheel routes killing an otherwise stout defense. The Rams, typically mediocre under Fischer, were still a major pain in the ass because Fischer knew to game plan around wheel routes. His backs ALWAYS had a receiving field day against Seattle, and I think that is where Belichick got clued in for his Super Bowl strategy.
Witherspoon, from the slot, may be the best answer for that yet. Not because he would primarily cover backs from the slot (he wouldn't), but because he reads and anticipates so well, that he could reliably leave his primary coverage of the slot receiver early and blow up the wheel routes to the running back as they happen. Not only that, but blow-up bubble screens aimed at the "X", take on jet sweeps, force the read option QB runs, switch on "rub" routes, etc. There is a ton of help that a slot corner with a Polamalu type anticipation could really help with.
Really, the guy is so good, he could do well at the boundary as well, but I think the team is better off using Michael Jackson there and utilizing Witherspoon at the slot.