My thought is that Issa should continue for awhile as he has been effective in building momentum.
If the story continues and it looks obvious that requests for documents and witness are not going to be complied with...that would be the time to announce a select committee.
To do so prematurely will look like a witch-hunt to the public (from whom support will be needed for this whole thing to really go anywhere).
That threshold has long since been crossed. White House, State, DOD and other agencies are withholding requested documents and preventing access to witnesses, per Issa.