A bench outfielder like Heisey, who is supposed to provide offense (not a backup shortstop), generally should not be 13 points below league average OBP for non-pitchers. So IMO the hypothetical .310 would not be adequate for Heisey, unless coupled with superb power hitting stats (always possible). He does provide good defense, but IMO if he doesn't hit he is a fifth outfielder, particularly on the Reds with OF questions.
But this is all theoretical. Because in fact Heisey did not hit .310 OBP last year. He had a .279 OBP and a 3.7 percent walk rate last year. If he does .310 or better next season (hopefully) we can resume the discussion about that number.
(Side note, of interest, when comparing only non-pitchers' hitting stats in both leagues, the AL has a .406-.401 edge in slugging, but the NL has a slight lead in OBP (2013)).