Does fixing cognitive biases do more harm than use? The Kahneman–Tversky interpretation of biases as deviation from rationality was challenged by Gigerenzer on the basis that heuristics help making decisions and, thus, rational from the evolutionary point of view.

What drawbacks of "fixing" biases have been empirically confirmed? I mean, avoiding heuristic rules requires more cognitive efforts, which possibly affect really important decisions.

PS: This question was separated from Are cognitive biases hardwired in the brain or they are acquired during lifetime?.
"Fixing" (compensating for) a cognitive bias means "improving the result", so by definition, the result is always better.  The drawback, as stated, is in the time and resources spent getting there.

Having said that, even when time spent is controlled for, there are scenarios where subjects make better decisions when they "trust their gut" rather than "think things through".

One example, cited here:

Students were asked to select a poster to take home, and then followed up a few weeks later to find out how happy they were with their selection.  The control group simply chose their preferred poster, while the experimental group was asked to provide reasons first.  The results: Students who provided reasons for their choice made different choices – preferring a different style of poster – and their satisfaction levels at follow-up were significantly lower.  Winner: Heuristics.

Another example described here:

Subjects were given descriptions of several cars, in the form of pros and cons, and asked to select the “best” one.  Most of the cars had an equal number of pros and cons or more cons than pros, but one of the cars had 75% pros and 25% cons, making it the correct choice.  Subjects in the experimental group were asked to give their decision some thought, while subjects in the control group were given the same amount of time to decide but asked to perform a distraction task during that time.  Clearly the experimental group had the advantage of dedicated mental resources to make a deliberated decision, while the control group would rely on heuristics.  The results: When the total number of pros and cons is small (4), conscious decisions are indeed  significantly better than unconscious decisions.  However, when the total number of pros and cons is relatively large (12), heuristics have the edge.

Also see: Bounded rationality.