Even with the F-15s the WZ-7 can fly higher than its operational ceiling. This picture was apparently taken from below the WZ-7. Also Japan's F-15s are really old I doubt they can still fly to their operational ceiling now, or, even if they could, how long they could stay at that height without breaking up into pieces mid-air.That's the advantage with operating F-15. F-18E/F's operational ceiling is too low to catching WZ-7 at the height it is flying.