For an api, given that we have
formatargvalue(*getargvalues, ...)
formatargspec(*getfullargspec, ...)
we might add
formatsignature(signature, ...)
with the default being the same as str(signature).

I checked the first 80 hits at
http://code.ohloh.net/search?s=formatargspec%28&p=7&pp=0&fl=Python&mp=1&ml=1&me=1&md=1&ff=1&filterChecked=true
(which I found with 'search code' on Google) and 10-15% used one of the extended args. Another at least 5% used [1:=1] to strip of '(' and ')', so any new function might have 'parens=True' option.

Idle uses the default formatargspec, like almost all other uses, and I presume str(sig) is the same except for where formatargspec is arguably wrong (bound methods).  A calltip is a tip, not a doc, so I am most concerned that parameter names not be misleading, as in pattern.match(string) having the parameter misnamed 'pattern' in the C code and I presume, in .signature (absent being fixed). But that is not .signature's fault.