In signal detection theory, d' (sensitivity) is computed in terms of z-scores, to allow comparison (subtraction) of hit rates from false alarm rates. Even so, the interpretation of the d' is not straightforward: I was not able to find any absolute guidelines in terms of how much d' needs to be for it to mean "good" sensitivity.
From eye-balling a bunch of papers, it seems that d' should be above 2 or at least 1.5, but it's not at all clear. The only absolute reference is of course d'=0, which corresponds to chance.
Also, at the group level, is a good enough (i.e. above chance) performance necessarily indicated by a significant 1-sample t-test against zero?