Do the showplan (any or all) events add more overhead than a tuning trace should have How long (order of magnitude) do the various showplan events take to record? Also is the time these events require counted as part of the "duration" column of the corresponding SQL query?
There is no way to tell you any exact order of magnitude - this will depend greatly on how many plans you are collecting, how big they are, etc.

That said, yes, capturing actual showplan information will absolutely have an impact on your tracing. This article demonstrates that and this article shows that it is still a problem with Extended Events as well.