There's an article in Editor and Publisher claiming that Barry Bonds' body armor actually gives him  mechanical advantage(s) in addition to the protective function and that his use of the armor is cheating on a scale far worse than Sosa's corked bat (if Sosa ever used it in a game aside from the one where it broke, as he claimed). There's an estimate in the article that the armor has added 75-100 HR to Bonds total that he wouldn't have hit out without the body armor.

For years, sportswriters remarked that his massive "protective" gear - unequaled in all of baseball -- permits Bonds to lean over the plate without fear of being hit by a pitch. Thus situated, Bonds can handle the outside pitch (where most pitchers live) unusually well. This is unfair advantage enough, but no longer controversial. However, it is only one of at least seven (largely unexplored) advantages conferred by the apparatus.
...
Bonds has worn some sort of front arm protection since 1992... It seems to have remained relatively the same [ed - since 1996] until—interestingly—2001, the year of his record 73 home runs, when an advanced model appeared made (apparently) of a new material.

The article is written by Michael Witte, a "mechanics consultant to a Major League baseball Team."

Turns out that Michael Witte is a mechanics consultant for our very own beloved Cardinals, whose path to employment by the Cardinals, LLC., is described in this article from the New Yorker a year ago.

Jeff Luhnow, the Cardinals' vice-president of player procurement, admits that Witte at first seemed to have "very little credibility," but he nevertheless put him on the payroll as a consultant.

My two cents: players shouldn't be allowed to wear body armor beyond the little shin guards and Witte probably won't be employed by the Cardinals for much longer.