Are you so sure that it "does not matter what [you] think"? The rest of this statement is based solely on "what you think" transpired in the, to date unreported, negotiations Boras had with teams interested in JD Drew that offseason. That's a rather large leap of faith, one that you are apparently comfortable making when it's about "what you think", so I contend that, to you, what you think does matter.
Furthermore, using the basis of your logic in defining "overpaid", every FA ever signed, with the exception of those that play for a discount at a preferred location, is "overpaid", because, to some degree, they sign for some amount of money more than another team offered. I know what you are getting at is the magnitude of overpayment, but that's not what was asked. What was asked is if he is overpaid at all, and the application of your basis for determination would lead one to conclude that virtually all FA are overpaid, which is not very useful, not when the core of the original question goes back to the idea of payment vs. production. This is why WAR is meaningful when addressing this question.