Another perspective to consider: if Spring Training offensive stats are meaningless -- batting vs. big leaguers trying new pitches or grips, or minor leaguers stressing to impress -- then so are early season stats.
Even the Red Sox dismiss certain metrics over others, like when they promoted Anthony to a higher level in the minors with a low batting average solely because of his hard-hit rates. Or when they started Campbell on Opening Day because he was the best minor league player last year.
Not all hitters flop like Holliday when first promoted to the majors. Sometimes, players enjoy more success seeing more strikes to swing at, under better lighting conditions, after better rest and nutrition from big league travel and accommodations, with bigger crowds cheering them on. At least, initially -- until pitchers (and advance scouts) find a hole in their swings to exploit.
Mayer's production at Worcester doesn't show struggles: 4 HRs, 22 RBI in 17 Games. He also has 16 Ks, so a similar profile to Story's start in Boston: 5 HR, 14 RBI, 25 Ks in 22 G.
Let's also not forget Mayer clearly outplayed Campbell in the infield in Florida, and was as good or better than Story, as well. He knows he's a big leaguer; there's just no openings for him in Boston yet...