Definitely worth flying 10,000 miles for PremEx, even at the cost (an e-fare to LHR would work, for <$500). Assuming he'll fly ~50,000 miles next year, just think of it as "buying" 5368 x 2 x 1.25 premier bonus = 13,420 miles, plus 37,500 additional miles next year (100% PremEx bonus vs. 25% Premier) plusany other PremEx goodies (especially easier upgrades and waitlists).