Its not so easy to measure, as the only real measurable is to have the exact same game released as both a licenced and not licenced theme.  Then the sales of the two can be compared.

As stated in the article, Stern and JJP wouldn't pay for a licence for no reason.  Stern has the experience to know its affect.  I heard this said many times: "I like Mustang, but I wouldn't buy it as I don't like the theme."

We often discuss if a licence sells more games, but one thing that is clear is THEMES do sell games.  Often good themes need to be licenced!