I should have given a little more detail in mentioning Adair's estimates. He calculated that a baseball spinning at 1800 rpm, which he claims is the highest rate pitchers actually can achieve, would have a Magnus force equivalent to 1/4th the weight of the ball for any pitch speed between 50 and 105 mph. He also states that he believes his estimate is accurate to within 25%. His believes his estimate is bolstered by noting that sidearm pitchers, who generate Magnus force in the horizontal plane, thus separating the effects of aerodynamics and gravity, can only achieve about 8" of horizontal break. That corresponds nicely to 1/4 of gravity.
Is he right? I think a number of MLB parks have the equipment in place to make the measurements routinely, so maybe we'll find out in the next year or so.