I am replying here instead of higher because I agree with mattnewport, but this is addressed to Annoyance. It is hard to for me to understand what you mean by your post because the links are invisible and I did not instinctively fill them in correctly.

Overconfidence is usually costlier than underconfidence.

As best as I can tell, this is situational. I think mattnewport's response is accurate. More on this below.

The cost to become completely accurate is often greater than the benefit of being slightly-inaccurate-but-close-enough.

It seems that the two paths from this statement are to stay inaccurate or start getting more efficient at optimizing your accuracy. It sounds too similar to saying, "It is too hard. I give up," for me to automatically choose inaccuracy. I want to know why it is so hard to become more accurate.

It also seems situational in the sense that it is not always, just often. This is relevant below.

When these two principles are taken into account, underconfidence becomes an excellent strategy.

In addition to mattnewport's comment about underconfidence implying non-optimal confidence, I think that building this statement on two situational principles is dangerous. Filling out the (situational) blanks leads to this statement:

If underconfidence is less costly than overconfidence and the cost of becoming more accurate is more than the benefit of being more accurate than stay underconfident.

This seems to work just as well as saying this:

If overconfidence is less costly than underconfidence and the cost of becoming more accurate is more than the benefit of being more accurate than stay overconfident.

Which can really be generalized to this:

If it costs more to change your confidence than the resulting benefit, do not change.

Which just leads us back to mattnewport's comment about optimal confidence. It also seems like it was not the point you were trying to make, so I assume I made a mistake somewhere. As best as I can tell, it was underemphasizing the two situational claims. As a result, I fully understand the request for more support in that area.

It also leaves potential in reserve in case of emergencies. As being accurately-confident tends to let others know what you can do, it's often desirable to create a false appearance.

Acting overconfident is another form of bluffing. Also, acting one way or the other is a little different than understanding your own limits. How does it help if you bluff yourself?

"Overconfidence and underconfidence both imply a non-optimal amount of confidence."

Not in the sense of logical implication. The terms refer to levels of confidence greater or lesser than they should be, with the criteria utilized determining what 'should' means in context. The utility of the level of confidence isn't necessarily linked to its accuracy.

Although accuracy is often highly useful, there are times when it's better to be inaccurate, or to be inaccurate in a particular way, or a particular direction.

"You seem to have a pattern of responding to posts with unsupported statements"

I can support my statements, and support my supports, and support my support supports, but I can't provide an infinite chain of supports. No one can. The most basic components of any discussion stand by themselves, and are validated or not by comparison with reality. Deal with it.

"that appear designed more to antagonize than to add useful information to the conversation"

They're crafted to encourage people to think and to facilitate that process to the degree to which that is possible. I can certainly see how people uninterested in thinking would find that unhelpful, even antagonizing. So?