I don't think Star Wars had anything to do with Jarre using a symphonic sound in the score, because The Prince and the Pauper was released in England roughly less than a month after Star Wars. Unless I am totally wrong (since I have no recording dates for reference), Jarre probably scored the film before the release of Star Wars. It is possible that Jarre thought a symphonic sound was appropriate, especially considering Legrand's and Schifrin's scores for the Musketeers films were somewhat symphonic (if not completely).
I agree he blew it out of the ballpark. Its a great score. Great themes, dramatic and very energetic. Ive only seen the film on first release so I have no idea if there's additional music for an expansion or not.
I would love an expanded edition of this, particularly if it has the kind of sonic improvements Swashbuckler had over its previous incarnation. The FSM album sounds good, but if I'm not mistaken, it was from an album master.