No data has come my way that compares the pay of promoted general counsels to the pay of newly hired general counsel. It makes sense, however, to me that the outsiders would make more.
If no lawyer within the company is good enough to fill an open general counsel position, the outsider enjoys more leverage when negotiating the compensation package. Secondly, someone from another company has often been a general counsel and knows about the perks of that office. As a third reason, executive search firms rarely spot in-house talent so they lure away other GCs who need enough of a compensation boost to make the transfer worthwhile; it jacks up the cost of recruitment by driving up fees (See my post of March 26, 2005 on executive search firms and compensation data.)